Question

A boat covers a certain distance downstream in 8 hours and comes back upstream in 10 hours. If the speed of the current be 1 km/hr, the distance (in km) of the one way journey is 

(a) 60 

(b) 70 

(c) 80 

(d) 90

Answer 1

(c) 80 km

Speed of boat in still water be u km/hr. Then, 

Distance covered downstream = (u + 1) × 8 = 8u + 8 

Distance covered upstream = (u – 1) × 10 = 10u – 10 

Since the distance both ways is the same,

8u + 8 = 10u – 10 ⇒ 2u = 18 ⇒ u = 9 km/hr

∴ Distance covered one way = (9 + 1) × 8 km = 80 km.