Question

A motor boat goes down the stream 30 km and again returns to the
starting point in a total time of 4 hours and 30 minutes. If the speed of the
stream is 5 km/hr, then find the speed of the motor boat in still water.​

Answer 1

Answer:

15 km/h

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Answer 2

≡QUESTION≡

A motor boat goes down the stream 30 km and again returns to the  starting point in a total time of 4 hours and 30 minutes. If the speed of the  stream is 5 km/hr, then find the speed of the motor boat in still water.​

                                                               

║⊕ANSWER⊕║

Let the speed of the boat in still water be “x” km/hr.

The speed of the boat downstream = (x + 5) km/hr

The speed of the boat upstream = (x - 5) km/hr

Time taken to travel downstream = 30/(x+5)

Time taken taken to travel upstream = 30/(x-5)

Equation formed :

$\frac{30}{x+5} + \frac{30}{x-5} = \frac{9}{2}$

Solving,

3x² – 40x – 75 = 0

3x² – 45x + 5x – 75 = 0

3x(x-15) + 5(x-15) = 0

(x-15)(3x+5) = 0

x = 15 or -5/3

-ve value is not considered

∴x = 15 km/hr