A jogger desires to run a certain course in 1/4 less time than he usually takes. By What per cent must be increase his average running speed to accopplish the goal?

50%
20%
25%
331/3

Correct Answer: 331/3

Explanation:

Let usual speed and usual time taken by the jogger are D and T, respectively.
Let his new speed be D'. Then,
DT = (3D'/4)T
⇒ D = 3D'/4
⇒ D' = 4D/3
Thus, he has to increased his speed by
[(4D/3 - D) / D] x 100% .
= 33¹/₃ %

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