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:

Answer: Option B

Let the required number of revolutions made by larger wheel be x.
Then, More cogs, Less revolutions (Indirect Proportion)
$$\begin{array}{rl}& \therefore 14:6::21:x\\ & \Rightarrow 14\times x=6\times 21\\ & \Rightarrow x=\frac{6\times 21}{14}\\ & \Rightarrow x=9\end{array}$$