Thanks to one of our A+ quiz takers for sending along the subject quiz. I always hold my breath until the A+ originator lets me know if I got it right.
"Yes, exactly. . .well done" was his reply to my solution to his quiz.
Below is the subject quiz. Please let me know how you work the problem.
I will post all correct answers or alternatively will send the solution privately to anyone who requests it.
Bridge Crossing Quiz
It is given that a bridge, like the one shown above, will collapse in 17 minutes.
Four people want to cross the bridge before it collapses. It is night & there is only one flashlight that is necessary to guide them on any crossing. Only two people can cross @ a time & the pair must walk @ the rate of the slower person's pace.
A takes 1 minute to cross.
B takes 2 minutes
C takes 5 minutes
D takes 10 minutes
How do these four people organize their crossing so that all cross the bridge in 17 minutes before it collapses?
What they have to do is as soon as first gets half way the next person starts, when second reaches half way the first will be off the bridge and the third starts when the third reaches half way the second one will be off and the forth will start, when the forth reaches half way the third will be off they all crossed within 10 minutes.
ReplyDeleteSomething is wrong. It takes D 10 minutes just by himself so we know it cannot be 10 minutes for all four of the bridge crossers. Also anyone on the bridge must have the flashlight so they can see. People half way across could not see without the flashlight. Please try again keeping the flashlight in mind.
DeleteHere's my solution:
ReplyDeleteA & B cross with flashlight in 2 mins.
A returns with the flashlight in 1 min.
C & D cross the bridge with flashlight in 10 mins.
Now B returns with the flashlight in 2 mins.
Finally A & B cross the bridge with flashlight once again in 2 mins . The four persons have now crossed the bridge safely in total time of 17 minutes prior to the collapse of the bridge.
Great job. PB is the first with the correct solution.
DeleteHi Doug,
ReplyDeletePlease find the answer below:
1. A and B cross take 2 minutes
2. A back take 1 minute =======> 2 + 1 = 3 minutes
3. C and D cross take 10 minutes ======> 3 + 10 = 13 minutes
4. B back takes 2 minutes =====> 13 + 2 = 15 minutes
5. A and B cross take 2 minutes ======> 15 + 2 = 17 minutes
I cheated!
ReplyDelete1+2= 1
1=1
5+10=10
2=2
1+2=2. 2+1+10+2+2 = 17
However, by the time I’d really figured it out, the bridge would have collapsed!
It's uncanny that the numbers work out to 17. Makes me wonder if SRK really got it & is just playing with us.
Delete