Help me in math please

Question

A ship traveling down stream on a river in 4 hours swims 10 km less than would traveling up stream in 6 hours. Find the speed of the ship if,on this river, in 15 hours it swims the same distance as as it would on the lake.

Answer 1

4 hours down stream goes 10 km less than up stream in 6 hours

numbers that work are ship speed = 5 kph, current speed = 0 kph

d=rt

6(5) = 30 k

4(5) = 20 km = 30-10 km