Let speed of the boat in still water be x km/h and speedof current be y km/h. Then, upstream speed = (x – y) km/h and down stream speed = (x + y) km/h Now,
15
(x−y)
+
21
2(x+y)
= 3
1
4
.... (i)
12
(x−y)
+
14
(x+y)
= 3 ..... (ii) From Equation (i) and (ii) x = 10 km/hr and y = 4 km/hr.