Manuel has a boat that can move at a speed of 15 km/h in still water. He rides 140 km downstream in a river in the same time it takes to ride 35km upstream. What is the speed of the river?

Answer: the speed of the river is 9km/h

Step-by-step explanation:

Let x represent the speed of the river current.

He rides 140 km downstream in a river in the same time it takes to ride 35km upstream. This means that his speed was higher when riding downstream and it was lower when riding upstream.

Assuming he rode in the direction of the river current when coming downstream and rode against the current when going upstream.

time = distance/speed

Manuel has a boat that can move at a speed of 15 km/h

His downstream speed would be

15 + x

time spent coming downstream would be

140 / (15 + x)

His downstream speed would be

15 - x

time spent going downstream would be

35 / (15 - x)

Since the time is the same, then

140 / (15 + x) = 35 / (15 - x)

Crossmultiplying

140 (15 - x) = 35 (15 + x)

2100 - 140x = 525 + 35x

140x + 35x = 2100 - 525

175x = 1575

x = 1575/175

x = 9