Math: a motorboat can cover a certain distance on a lake in 5 hours.

Question

a motorboat can cover a certain distance on a lake in 5 hours. the same distance can be covered by a raft travelling down the river in 20 hours. how long will it take the motorboat to cover that distance traveling downstream?

PLS HELP!!!

Answer 1

raft does the distance in 20 hours

motorboat does the same distance in 5 hours

let's say the distance was 20 hours

then the current was 1 mph and the boat speed was 4 mph

the boat plus current speed = 4+1 = 5mph

at 5 mph it takes the boat 20/5 = 4 hours to cover the same distance

try the same problem with distance - 40 miles

then the current speed = 40/20 = 2 mph

the boat speed = 40/5 = 8 mph

downstream 8+2 = 10 miles per hour

40/10 = 4 hours to cover the same distance

answer is always 4 hours, regardless of the distance