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**Problem :**

U2 has a concert that starts in 17 minutes and they must all
cross a bridge to get there. All four men begin on the same side of the bridge.
You must help them across to the other side. It is night. There is one
flashlight. A maximum of two people can cross at one time. Any party who
crosses, either 1 or 2 people, must have the flashlight with them.

The flashlight must be walked back and forth. It cannot be thrown and other
tricks like that are not needed to solve the problem. The solution is simply a
matter of allocating resources in a certain order. Each band member walks at a
different speed. A pair must walk together at the rate of the slower man's
pace:

Bono: 1 minute to cross

Edge: 2 minutes to cross

Adam: 5 minutes to cross

Larry: 10 minutes to cross

Edge: 2 minutes to cross

Adam: 5 minutes to cross

Larry: 10 minutes to cross

__For example__:
if Bono and Larry walk across first, 10 minutes have elapsed when
they get to the other side of the bridge. If Larry then returns with the
flashlight, a total of 20 minutes have passed and you have failed the mission.

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**Solution:**

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)————->| ( AB comes and A returns)

ACD |<——-A-(1min)————--|B

A || ——-CD(10min)———>| BCD (CD come and B returns)

A || |<——-B-(2min)————--|

AB |—————————–-------|ABCD (AB come)

If you count all the trips ie. 2+1+10+2+2, it will be 17 minutes

CD |——-AB(2min)————->| ( AB comes and A returns)

ACD |<——-A-(1min)————--|B

A || ——-CD(10min)———>| BCD (CD come and B returns)

A || |<——-B-(2min)————--|

AB |—————————–-------|ABCD (AB come)

If you count all the trips ie. 2+1+10+2+2, it will be 17 minutes

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