Question

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

Answer 1

Let us assume the speed of boat in still water is x and speed of current is y.

Hence, downstream speed = x + y and upstream speed = x – y

Case (i): $time=\frac{distance}{speed}$ [downstream]

$2=\frac{20}{x+y}$

2 (x+y) = 20

2x + 2y = 20

x + y = 10 -------(i)

Case (ii): $time=\frac{distance}{speed}$ [upstream]

$2=\frac{4}{x-y}$

2(x -y) = 4

x -y = 2 -------(ii)

Now, add eq.(i) ad eq. (ii);

2x = 12

x = 6

Substituting, the value of x in eq.(i)

x + y = 10

y = 10 - 6 = 4

Thus, speed of rowing in still water = 6 km/hr and speed of current = 4 km/hr