A wheel that has 6 cogs is meshed with a larger wheel of 14 cogs. If the smaller wheel has made 21 revolutions, what will be the number of revolutions made by the larger wheel?
Let the number of revolutions made by the larger wheel be $x$
More cogs, less revolutions (Indirect proportion)
Hence we can write as
(cogs) 6 : 14 :: $x$ : 21
$\Rightarrow 6 \times 21 = 14 \times x\\$
$\Rightarrow 6 \times 3 = 2 \times x\\$
$\Rightarrow 3\times 3 = x\\$
$\Rightarrow x = 9$