Question

It takes a ship 3 hours to cover 72 km with the current and 4 hours against the current. Find the speed of the ship in still water and the speed of the current.

Answer 1

Let 'u' be the speed of boat in still water and 'v' be the speed of water current.

Now, in upstream i.e, u-v,
boat covers a distance of 72 Km in 4 hours

And, in downstream i.e, u+v
boat covers a distance of 72 Km in 3 hours

using the formula,

speed = distance÷time
speed in upstream = 72 ÷ 4 = 18 km/hr
speed in downstream = 72 ÷ 3 = 24 km/hr

u + v = 24
u - v = 18

solving the equations using substitution method,

u = 24 - v

substitute this value of u to the other equation to get,

24 - v - v = 18
24-2v=18
-2v=-6
v = -6÷-2
v = 3 km/hr

substitute this value of v to first equation,

u = 24 - v
u = 24 - 3
u = 21 km/hr

speed of boat in still water is 21 km/hr and the speed of water current is 3 km/hr
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Hope it helps!