“A body covers a distance of 20 m in 7th s and 24 m in the 9th s. How much shall it cover in 15s ?” – Raghav posted this question.

Answer: From the eqn for displacement in the nth second,

For the displacement in the 7th sec, we get,

…………………….. (1)

For the displacement in the 9th sec,

……………………..(2)

(2)-(1) gives

24-20 = 2a

or a = 2 m/s^{2}

Substituting for a in (1),

u = 7 m/s

Substituting, u= 7 m/s, a = 2 m/s^{2} and t = 15 sec in the eqn,

we get S= 7 x 15 + 0.5 x 2x 15 x 15 = 105+225=**330m**

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