Question

Ritu can row downstream 20 km in 2 hours and upstream 4 km in 2 hours. Find her speed of rowing in still water and the speed of the current.

Answer 1

Given:

• Ritu can row 20 km downstream in 2 hours.
• Ritu can row 4 km upstream in 2 hours.

Step-wise calculation:

1. Speed downstream with the current 

= `"Distance"/"Time"` 

= `(20  km)/(2  hrs)` 

= 10 km/hr

2. Speed upstream against the current 

= `"Distance"/"Time"` 

= `(4  km)/(2  hrs)` 

= 2 km/hr

Let:

Speed of the boat in still water = x km/hr.

Speed of the current = y km/hr.

Using the relations:

Speed downstream = (x + y) km/hr.

Speed upstream = (x – y) km/hr.

From the above data: 

x + y = 10   ...(i)

x – y = 2   ...(ii)

Adding (i) and (ii):

2x = 12

x = 6 km/hr

Substituting x = 6 in equation (i):

6 + y = 10

y = 4 km/hr

Speed of rowing in still water = 6 km/hr.

Speed of the current = 4 km/hr.