21:56 is the same to 21/56 so just think of it as a fraction. So the two numbers are multiples of 7 so one possible answer could be 3/8 which is 3:8 then you could multiply both of those by the same number so by 2 it would be 6:16 and by 3 it would be 9:24. Those are all equivalent to the ratio 21:56.
