Question

MATH WHAT IS THIS!!!

Four people need to cross a dark river at night.They have only one torch and the river is too risky to cross without the tourch. if all people cross simultanoesly then torch light wont be sufficient.Speed of each person of crossing the river is different.cross time for each person is 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the river ?

Answer 1

17 minutes

Explanation:

Let A be the person who takes 1 min,

B who takes 2 mins, C who takes 7 mins and D who takes 10 mins

A and B cross the river in 2 mins, A comes back in 1 min

C and D cross the river in 10 mins, B comes back in 2 mins

A and B cross the river in 2 mins

Total time:

2 + 1 + 10 + 2 + 2 = 17 minutes

Answer 2

17 minutes i had this same answer and question 2day, so ur lucky.

Explanation:

The initial solution most people will think of is to use the fastest person as an usher to guide everyone across. How long would that take? 10 + 1 + 7 + 1 + 2 = 21 mins. Is that it? No. That would make this question too simple even as a warm up question.

To reduce the amount of time, we should find a way for 10 and 7 to go together. If they cross together, then we need one of them to come back to get the others. That would not be ideal. How do we get around that? Maybe we can have 1 waiting on the other side to bring the torch back. Ahaa, we are getting closer. The fastest way to get 1 across and be back is to use 2 to usher 1 across. So let's put all this together.

1 and 2 go cross

2 comes back

7 and 10 go across

1 comes back

1 and 2 go across (done)

Total time = 2 + 2 + 10 + 1 + 2 = 17 mins