Interview Questions And Answers 
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Four people need to cross a rickety rope bridge to get back to their camp at night. Unfortunately, they only have one flashlight and it only has enough light left for seventeen minutes. The bridge is too dangerous to cross without a flashlight, and it’s only strong enough to support two people at any given time. Each of the campers walks at a different speed. One can cross the bridge in 1 minute, another in 2 minutes, the third in 5 minutes, and the slow poke takes 10 minutes to cross. How do the campers make it across in 17 minutes?
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Let’s name the as A,B,C & D and let’s assume that they take 1,2,5 and 10 minutes respectively. Here is how they should cross the bridge.
ABCD |—————————–|
CD |——-AB(2min)————->|
CD || BCD
A || ABCD
If you count all the trips ie. 2+1+10+2+2, it will be 17 minutes
– 1, 2, 5, 10
1, 2 — 5, 10
2 — 1, 5, 10
2, 5, 10 — 1
5, 10 — 1, 2
1, 2, 5, 10 –
A(1), B(2), C(5), D(10)
1. A & B cross together. They take 2 minutes
2. A come back to start point. He takes 1 more min.
3. So far 3 minutes invested in total
4. C and D start together. Reach in 10 minutes. So total 13 minutes invested so far
5. B who is at the far end, take the torch and comes back to start point. He needs 2 min
6. So far far 15 min invested
7. A and B start together, reach the far end in 2 min
8. Total 17 min invested.
Done !!!
Its easy…..
a:-1 min
b:-2 mins
c:-5 mins
d:-10 mins
Ist Pass :- a,b–> takes 2 mins for inbound crossing and 1 min for a outbound leaving b at the other end
Total mins for Inbound and outbound :-
inbound trip :-2 mins
outbound trip:-1 min
———————
total :- 3 mins
————————–
2nd Pass :- c,d–> takes 10 mins for inbound crossing and 2 min for a outbound leaving both c,d at the other end and getting back b.
Total mins for Inbound and outbound :-
inbound trip :-10 mins
outbound trip:-2 min
———————
total :- 12 mins
————————–
3rd Pass :- a,b–> takes 2 mins for inbound crossing and no outbound trip
Total mins for Inbound and outbound :-
inbound trip :-2 mins
outbound trip:-0 min
———————
total :- 2 mins
————————–
got it !!!…..12+3+2=17mins
A(1), B(2), C(5), D(10)
1. A & B cross together. They take 2 minutes
2. A come back to start point. He takes 1 more min.
3. So far 3 minutes invested in total
4. C and D start together. Reach in 10 minutes. So total 13 minutes invested so far
5. B who is at the far end, take the torch and comes back to start point. He needs 2 min
6. So far far 15 min invested
7. A and B start together, reach the far end in 2 min
8. Total 17 min invested.
First the fourth on can cross along with third one who takes 10 &5 min respectively,So first third then fourth will go So for both time will be 10 min
Now first and then second one will go who takes 2 & 1 min respectively. So in combinedly it will take 2 min So total=12min As from single flashlight 2 person can see
Here i am trying to address the flashlight problem as well since it is dangerous to cross without the flashlight.
1. D(10 min) and C(5 Min) will start walking. D will hold the flash light.
2. By the time D will get to the middle, C will get off the bridge. Total Time So Far = 5 min
3. B will get on the bridge, D stays at middle of the bridge, helping B with Flash light untill B cross the bridge. Total Time (5 + 2 min of B) = 7 Min
4. A will get on the bridge, D still stays at middle of the bridge, Helping A with flash light untill A cross the bridge. Total Time (5 + 2 (B) + 1 (A)) = 9 min.
5. D will start walking and will be off the bridge in 5 min since he was at Halfway already. Total Time ( 5 + 2 + 1 + 5 ) = 13 min.
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