Question

Determine the average speed of the current in a river if a boat moves at 20 miles/hour in still water and takes 3 hours to travel 78 miles downstream

Answer 1

The speed of the current in a river is 6 miles per hour

Solution:

Given that,

Speed of boat in still water = 20 miles per hour

Time taken = 3 hours

Distance downstream = 78 miles

To find: Speed of current

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, speed downstream is given as:

$\text{ speed downstream } = \frac{distance}{time} = \frac{78}{3}\\\\\text{ speed downstream } = 26 \text{ miles per hour }$

We know that,

Speed downstream = (u + v)

26 = 20 + v

v = 26 - 20

v = 6 miles per hour

Thus speed of the current in a river is 6 miles per hour