A rectangle has a perimeter of 30 centimetres, and one of its sides measures 9 centimetres. How long is the side next to it?

Explication

Half the perimeter, 15 centimetres, holds one length together with one width, so the neighbouring side is 15 minus 9. Taking 9 away from the whole 30 leaves 21 and forgets that a perimeter carries two copies of each measurement.