Logic Quiz

Southie · August 4, 2015, 6:48am

Re: Logic Quiz

The answers to these questions are needed. Else the problem statement is incomplete

1) How many torches are there?
2) if onky one, does this mean that folks on the bridge - max of 2- have to be at same spot with torch. OR. Can you have the torch be present ANYWHERE in the bridge, with the two persons having the freedom to be in separate locations along the bridge.

I will assume one torch. And the max of 2 persons having freedom to be in separate locations along the bridge.

Southie · August 4, 2015, 6:55am

Re: Logic Quiz

16.

C and D set out first. D carries the torch. As soon as C reaches after 9 minutes, A sets out. A and D reached the other end at same time at t equals 12 min. D passes the torch to A at t equals 12 and steps off the bridge. A stays on the bridge near the destination side. At t equals 12 B sets off for the other side. At t equals 16 B reaches the destination. A and B step off the bridge at exactly 16 minutes.

And the fat lady bellowed.

psyah · August 4, 2015, 10:24am

Re: Logic Quiz

I don't think staying on the bridge is allowed. Likewise, I don't think walking halfway is allowed either. But that is an incomplete problem statement for you...

Southie · August 4, 2015, 10:57am

Re: Logic Quiz

Yes problem statement is not clear. Since it was not specified staying on bridge not allowed tbis wouod be valid solution.

I will now solve for case where staying on bridge not allowed. Or walking halfway not allowed. In other words have to go full from one side to other.

I assume exchanging torch allowed.

psyah · August 4, 2015, 11:00am

Re: Logic Quiz

With your latest assumptions the lowest I made it was 18 mins.

Southie · August 4, 2015, 2:00pm

Re: Logic Quiz

^ haven't read above post in case that has solution.

Here is solution for assumptions in previous post.

T=0. C And D set out. D has torch
T =9 C reaches. A starts
T=12. D and A reach. A takes torch. And turns around to go to start. B sets out.
T= 16. B reaches. A is 2 min from desitniation.
T =18 A reaches.

Answer 18 sec.

Southie · August 4, 2015, 2:01pm

Re: Logic Quiz

I had.a.feeling u had written the answer. I had scribbled out the possibilitues. And cqm3 with same answer.

MustafaQuraishi · August 4, 2015, 4:11pm

Re: Logic Quiz

Dhagga For Bongi pepss !!!

I sorta agree with with SID_NY but he might not be considering the light source.

Assuming that when the people start to walk in a given direction they have to continue until they get to the other side. They can’t stop on the bridge.

Then at t=0 D and B leave together - the candle is with D.
At t=4, B should be across so A can follow on to the bridge
At t=7, C can set off since only D remain on the bridge
However by t=12, C will have only completed 5/9th of its journey and there would be no light source
So A comes to the rescue and goes back collecting the light from D when it completes its journey at t=12
So A goes to the other end with candle in hand and comes back, the double trip taking 6 mins so in the last 2 minutes only A remains on the bridge. Which makes t=18

However if the people do not have to go end to end on the bridge then the time can be reduced to 16 mins, but this requires A to stop midway and align his return with C.

I thought i had cleared these assumptions in the update but i guess they weren’t cleared.

There is only one light source. Only one. there is no other option or no hidden way of getting another light source
A person can’t cross the bridge without a light source. Most of you people tried to gave the light source to D and paired it with another person (C). But in this case, C would only be able to cross the birdge at the speed of D since D is the one who is holding the light source. C can’t walk past D because if he does, he would be leading and wouldn’t have the light source to see. Similarly, If D is in the middle of the bridge with light source, another person from another end can’t start because he wouldn’t have the light source. The people at the other side has to wait for someone to bring the light source back to them.

To simplify,

A pair can only cross the bridge at the speed which is slowest between them. i.e. if A and D are crossing the bridge then A’s speed is irrelevant since D is walking at slower speed.
A single person can’t start the walk without having the light source. Consider the bridge as river and light source as boat. The speed of the boat would be the rowing speed of the slowest person.
There is only one light source. If A and D is carrying it and they reached the other end, then one of them has to bring the light source to the starting end to bring other people

I hope its clear now.

psyah · August 4, 2015, 6:03pm

Re: Logic Quiz

MustafaQuraishi:

I thought i had cleared these assumptions in the update but i guess they weren't cleared.

There is only one light source. Only one. there is no other option or no hidden way of getting another light source

A person can't cross the bridge without a light source. Most of you people tried to gave the light source to D and paired it with another person (C). But in this case, C would only be able to cross the birdge at the speed of D since D is the one who is holding the light source. C can't walk past D because if he does, he would be leading and wouldn't have the light source to see. Similarly, If D is in the middle of the bridge with light source, another person from another end can't start because he wouldn't have the light source. The people at the other side has to wait for someone to bring the light source back to them.

To simplify,

A pair can only cross the bridge at the speed which is slowest between them. i.e. if A and D are crossing the bridge then A's speed is irrelevant since D is walking at slower speed.

A single person can't start the walk without having the light source. Consider the bridge as river and light source as boat. The speed of the boat would be the rowing speed of the slowest person.

There is only one light source. If A and D is carrying it and they reached the other end, then one of them has to bring the light source to the starting end to bring other people

I hope its clear now.

Brother MustafaQureshi

Thanks for this explanation of the rules. It is important to make everything clear otherwise human minds will work with anything. Quite clearly the idea that the light source is needed to move and the slowest person is the pacemaker was impossible for us to know ... But it is clear that we need to rethink this now.

SID_NY · August 4, 2015, 6:05pm

Re: Logic Quiz

MustafaQuraishi:

I thought i had cleared these assumptions in the update but i guess they weren't cleared.

There is only one light source. Only one. there is no other option or no hidden way of getting another light source

A person can't cross the bridge without a light source. Most of you people tried to gave the light source to D and paired it with another person (C). But in this case, C would only be able to cross the birdge at the speed of D since D is the one who is holding the light source. C can't walk past D because if he does, he would be leading and wouldn't have the light source to see. Similarly, If D is in the middle of the bridge with light source, another person from another end can't start because he wouldn't have the light source. The people at the other side has to wait for someone to bring the light source back to them.

To simplify,

A pair can only cross the bridge at the speed which is slowest between them. i.e. if A and D are crossing the bridge then A's speed is irrelevant since D is walking at slower speed.

A single person can't start the walk without having the light source. Consider the bridge as river and light source as boat. The speed of the boat would be the rowing speed of the slowest person.

There is only one light source. If A and D is carrying it and they reached the other end, then one of them has to bring the light source to the starting end to bring other people

I hope its clear now.

With all these explanations the answer that I came up with is "31" assuming there is one light source and whoever is crossing the bridge must either himself or his partner has to carry it. But i still think its a very inefficient number so there gotto be a shorter number than this but cant think of anything right now.

MustafaQuraishi · August 4, 2015, 6:05pm

Re: Logic Quiz

apologies, if i wasn't clear. let me know if you guys have more questions.

MustafaQuraishi · August 4, 2015, 6:08pm

Re: Logic Quiz

Please explain your 31 number

psyah · August 4, 2015, 6:09pm

Re: Logic Quiz

I make it D with A, A returns, C with A, A returns, B with A ...

12+3+9+3+4=31

Ans=31

psyah · August 4, 2015, 6:11pm

Re: Logic Quiz

I'm not impressed with this puzzle ... The rules suppress any complexity and novel solutions.

SID_NY · August 4, 2015, 6:15pm

Re: Logic Quiz

A+B with B holding torch, reached in 4 min. = 4 min
A returns with torch. = 3min
A+C with C holding torch = 9 min
A returns again = 3min
A+D with D holding torch = 12 min
total = 31 min.

MustafaQuraishi · August 4, 2015, 6:22pm

Re: Logic Quiz

so far @psyah and @SID_NY gave the answer 31

Waiting for others before i give the optimal solution.

Southie · August 4, 2015, 7:09pm

Re: Logic Quiz

The assumptions were not clear. For ex it was not clear that each person needed the light source. As stated originally ONE light source had to be on the bridge with MAX OF TWO people at a time.

Even then there were several interpretations possible. Can one person wait near destination. So sold provided based on that. And another based on ni u can't wait. U have turn around and go back all the way.

With these new requirements will go back and provide solution in 8 hours or so won't look at other posts 5ilk them.

lander · August 5, 2015, 1:27am

Re: Logic Quiz

27

A+B = 4
A back = 3
C+D = 12
B back = 4

A+B = 4

Total = 27

Eastern_Dresses · August 5, 2015, 1:33am

Re: Logic Quiz

With this new explanation. My answer would be 27. Earlier, I was assuming that the slow person will just stop at interval and wait for the torch to be brought back on the bridge.

Anyway, this one is easier to explain. Here is the explanation for my answer
A and B go to the other side with the torch - Time 4 min
A returns with the torch - Time 3 min
C and D go to the other side with the torch - Time 12 min
B returns with the torch - Time 4 min
A and B go to the other side with the torch - Time 4 min

Total time - 27 min

Southie · August 5, 2015, 4:02am

Re: Logic Quiz

31 min

A is fastest. Accompanies first person. Gets back. Next person. Gets back. Next person. Done.

12 plus 9 plus 4 plus 2 trips back for A or 2 times 3

31

Doesn't matter in what order B C and D make the trip.

The fat lady had lost her voice by now.