arithmetic - a wheel that has 9 cogs is meshed with a larger wheel of 21 cogs when the smaller wheel has made 42 revolutions then the number of revolutions made by the larger wheel is - skillgun

Let the required number of revolutions made by larger wheel be x.
Then, More cogs, Less revolutions (Indirect Proportion)
=> 21 : 9 :: 42 : x
=> 21 x x = 9 x 42
=> x = 9 x 42 /21
=> x = 18