Question

A boat running downstream covers a distance of 8.4 km in 12 minutes. If the same boat at same speed and same distance require 15 minutes for upstream, then find the speed of the stream.

• A. 4.2 kmph
• B. 4.6 kmph
• C. 5 kmph
• D. 5.6 kmph

Answer 1

Answer:

A. 4.2 kmph

Let the speed of the boat in still water be x kmph, speed of stream be y kmph

$Speed=\frac{Distance}{Time}$

Downstream speed of boat, x + y = $\frac{8.4}{(\frac{12}{60})}$

$=\frac{8.4}{12}\times{60}=42kmph$ ---- (1)

Upstream speed of boat, x – y = $\frac{8.4}{(\frac{15}{60)}}$

$=\frac{4.2}{15}\times{60}=33.6 kmph$ ---- (2)

On solving both the equations,

⇒ 2x = 75.6

⇒ x = 37.8 kmph

Put the value in Equation (1)

⇒ 37.8 + y = 42

⇒ y = 42 – 37.8 = 4.2 kmph

∴ Speed of the stream is 4.2 kmph