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1. A wheel that has 6 cogs is meshed with a larger wheel of 14 cogs. When the smaller wheel has made 21 revolutions, then the number of revolutions mad by the larger wheel is:

A. 4

B. 9

C. 12

D. 49

Solution:

Let the required number of revolutions made by larger wheel be x.
Then, More cogs, Less revolutions (Indirect Proportion)

\begin{aligned} ∴ 14 : 6 :: 21 : x \\ \Rightarrow 14 \times x = 6 \times 21 \\ \Rightarrow x = \frac{6 \times 21}{14} \\ \Rightarrow x = 9 \end{aligned}

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