The speed of boat in still water is 24 kmph. It goes 210 km upstream and returns downstream to the original point In 40hours. The speed of the stream is?

# The speed of boat in still water is 24 kmph. It goes 210 km upstream and returns downstream to the original point In 40hours. The speed of the stream is?

1. A

18

2. B

24

3. C

36

4. D

72

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### Solution:

Solution: Let the speed of  the stream = x km/hr

Speed of  the boat in still water = 24 kmph

In down stream $=\left(24+x\right)\text{\hspace{0.17em}\hspace{0.17em}}kmph$

In up stream $=\left(24-x\right)\text{\hspace{0.17em}\hspace{0.17em}}kmph$

According to the given information

$\frac{210}{\left(24+x\right)}+\frac{210}{\left(24-x\right)}$=40

$210\left[\frac{24+x+24-x}{576-{x}^{2}}\right]=40$

$\frac{48}{576-{x}^{2}}=\frac{4}{21}$

$576-{x}^{2}=252$

${x}^{2}=324$

$x=\sqrt{324}\text{\hspace{0.17em}}=18$

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