Pinky can swim 27 km downstream or 15 km upstream in 3 hours. What is the velocity of the current?

Let's assume the speed of the boat in still water and the velocity of the current are 'B' and 'C' respectively.

Pinky can swim 27 km downstream or 15 km upstream in 3 hours.

$$\frac{27}{B+C}\ =\ 3$$

$$\frac{9}{B+C}\ =\ 1$$

B+C = 9    Eq.(i)

$$\frac{15}{B-C}\ =\ 3$$

$$\frac{5}{B-C}\ =\ 1$$

B-C = 5    Eq.(ii)

Add Eq.(i) and Eq.(ii).

B+C+B-C = 9+5

2B = 14

B = 7 km/h

Put the value of 'B' in Eq.(i).

7+C = 9

C = 9-7 = 2

Velocity of the current = C = 2 km/h