Question

A motorboat travels 165 kilometers in 3 hours going upstream and 510 kilometers in 6 hours going downstream. What is the rate of the boat in still water and what is the rate of the current?

Answer 1

Rate of the boat in still water is 70 km/hr and rate of the current is 15 km/hr

Solution:

Given that,

A motorboat travels 165 kilometers in 3 hours going upstream and 510 kilometers in 6 hours going downstream

Therefore,

Upstream distance = 165 km

Upstream time = 3 hours

Find upstream speed:

$speed = \frac{distance}{time}\\\\speed = \frac{165}{3}\\\\speed = 55$

Thus upstream speed is 55 km per hour

Downstream distance = 510 km

Downstream time = 6 hours

Find downstream speed:

$speed = \frac{510}{6}\\\\speed = 85$

Thus downstream speed is 85 km per hour

If the speed of a boat in still water is u km/hr and the speed of the stream is v km/hr, then

Speed downstream = u + v km/hr

Speed upstream = u - v km/hr

Therefore,

u + v = 85 ----- eqn 1

u - v = 55 ----- eqn 2

Solve both

Add them

u + v + u - v = 85 + 55

2u = 140

u = 70

Substitute u = 70 in eqn 1

70 + v = 85

v = 85 - 70

v = 15

Thus rate of the boat in still water is 70 km/hr and rate of the current is 15 km/hr