Question

A boat takes 2 hours 30 minutes to travel 4 km down and 4 km up

Answer 1

Let s be the speed of the boat in still water. Then:

4/s+3 + 4/s-3=5/2

4(2)(s-3)+4(2)(s+3)=5(s+3)(s-3)

8s-24+8s+24=5s²-45

16s=5s²-45

5s²-16s-45=0

(5s+9)(s-5)=0

s=5

Now, let the speed of the current be 1. Then:

4/5+1 + 4/5–1=4/6+4/4

=1 2/3 hours