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1. A can go round a circular path 8 times in 40 minutes. If the diameter of the circle is increased to 10 times the original diameter, then the time required by A to go round the new path once, travelling at the same speed as before, is :

A. 20 min

B. 25 min

C. 50 min

D. 100 min

Solution:

Let original diameter be d metre
Then, its circumference =

(π d)

metres
Time taken to cover

(8 π d)

m = 40 min
New diameter =

(10 d)

m
Then, its circumference =

(π \times 10 d)

m
∴ Time taken to go round it once :

\begin{aligned} = (\frac{40}{8 π d} \times 10 π d) min \\ = 50 min \end{aligned}

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