It takes 8 days for a truck to cross the desert. Each truck can carry gasoline for only 5 days.?

Question

What is the smallest number of trucks to start the trip in order for one truck to get across and the others to return?

Please explain how you got the answer. Thanks

Answer 1

Terrific question.

There must be a lot of theory in it.

I'll try.

4 trucks.

Truck 1 turns back after 1 day.

Truck 2 turns back after 2 days.

Truck 3 turns back after 3 days.

Truck 4 drives across.

Truck 1 gives a day of fuel to each of trucks 2,3,4

Truck 2 gives a day of fuel to each of trucks 3 and 4.

Truck 3 gives a day of fuel to truck 4.

Truck one uses 2 days fuel and gives away 3 days fuel (one days fuel each to trucks 2,3,4)

Truck two uses 4 days fuel and is given one days fuel (by truck 1) and gives away 2 days fuel (one days fuel each to trucks 3,4)

Truck 3 uses 6 days fuel and is given 2 days fuel (one days fuel each from trucks 2,3) and gives away one days fuel (to truck 4)

Truck 4 uses 8 days fuel and is given 3 days fuel (one days fuel each from trucks 1,2 3)

Method : I just tried. Took ages.