A) \[30\,km{{h}^{-1}}\]
B) \[13\,km{{h}^{-1}}\]
C) \[60\,km{{h}^{-1}}\]
D) \[40\,km{{h}^{-1}}\]
E) \[20\,km{{h}^{-1}}\]
Correct Answer: A
Solution :
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 \[{{v}_{2}}=?\] \[40=\frac{2\times 60\times {{v}_{2}}}{60+{{v}_{2}}}\] \[{{v}_{2}}=30\text{ }km{{h}^{-2}}\]
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