Question

A car moves a distance of $200\, m$. It covers first half of the distance at speed $ 60\,\,km{{h}^{-1}} $ and the second half at speed $v$. If the average speed is $ 40\,\,km{{h}^{-1}}, $ the value of $v$ is

• A. $ 30\,km{{h}^{-1}} $
• B. $ 13\,km{{h}^{-1}} $
• C. $ 60\,km{{h}^{-1}} $
• D. $ 40\,km{{h}^{-1}} $
• E. $ 20\,km{{h}^{-1}} $

Answer 1

Answer: (A)

Average velocity $ =\frac{2{{v}_{1}}{{v}_{2}}}{{{v}_{1}}+{{v}_{2}}} $
Given $ {{v}_{av}}=40\text{ }km{{h}^{-1}} $
$ {{v}_{1}}=60\text{ }km{{h}^{-1}} $ and $ {{v}_{2}}=? $
$ 40=\frac{2\times 60\times {{v}_{2}}}{60+{{v}_{2}}} $
$ {{v}_{2}}=30\text{ }km{{h}^{-2}} $