can someone pls tell me how to solve for the time LIKE HOW THE EQUATION WORKS? PLSSSSSSSSSS

Question

 A man goes fishing in a river and wants to know how long it will take him to get 10km upstream to his favourite fishing location. The speed of the current is 3 km/hr and it takes his boat twice as long to go 3km upstream as is does to go 4km downstream. How long will it take his boat to get to his fishing spot?

Answer 1

Let the speed of the boat in still water be s. Then, if the speed of the current is c, the speed of the boat is s - c upstream and s + c downstream.

c = 3

Thus, the time to go 3 km upstream is 3/(s - 3), and the time to go 4 km downstream is 4/(s + 3)

We are given that it takes twice as long to go upstream, so

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

Cross-multipling, we get, 8s - 24 = 3s + 9

5s = 33

s = 33/5

Then, to get 10 km upstream, it will take 10/(33/5 - 3) = 10/(18/5) = 50/18 = 2 7/9 hr