A boat covers a certain distance downstream in 8 hours and comes back

1.	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» (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.

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»

(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.

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