If a boy running against a wind with speed $2 m / s$ reaches the finish line of a $100 m$ race in 15 seconds. How long will he take to complete the race in absence of wind.

11 s

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11.5 s

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11.8 s

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12 s

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Solution

The correct option is B $11.5 s$
If speed of boy is $v_{b}$ and speed of wind is $v_{w} = 2 m / s$
Then, $v_{b} - 2 = \frac{100}{15} \Rightarrow v_{b} = \frac{130}{15} m / s$
time in absence of wind, $t = \frac{100}{v_{b}} = \frac{100 \times 15}{130}$
$t = 11.5 s$

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