A boat covers a certain distance upstream in 15 hours and covers the same distance downstream in 9 hours. The boat covers x km in still water in 5 hours and also covers (x + 30) km downstream in 5 hours. Find the speed of the boat in still water.
a) 18 km/hr
b) 20 km/hr
c) 30 km/hr
d) 24 km/hr
e) None of these
Sol:
Speed of the boat in still water = B km/hr
Speed of the current = C km/hr
(B + C) * 9 = (B – C) * 15
9B + 9C = 15B – 15C
6B = 24C
B/C = 4/1
Speed of the boat in still water = 4y km/hr
Speed of the current = y km/hr
x/4y = 5
x = 20y
(x + 30)/(4y + y) = 5
(20y + 30)/5y = 5
20y + 30 = 25y
5y = 30
y = 6 km/hr
Speed of the boat in still water = 4 * 6 = 24 km/hr
