Well since your width is fixed at 20 cm (and there are two of these in a rectangle) this will account for 40cm of your perimeter.

Take 40 cm away from 88 cm and you get 44 cm (this accounts for both your lengths, so divide this by two) 44/2 = 22cm. This tells you that a rectangle of width 20 cm and length 22cm will EQUAL 88cm, but you want to know what length will give you a perimiter > 88cm. So any length > 22cm (but not equal to 22cm) will give you a perimeter GREATER than 88cm.

I hope this helps.

Take 40 cm away from 88 cm and you get 44 cm (this accounts for both your lengths, so divide this by two) 44/2 = 22cm. This tells you that a rectangle of width 20 cm and length 22cm will EQUAL 88cm, but you want to know what length will give you a perimiter > 88cm. So any length > 22cm (but not equal to 22cm) will give you a perimeter GREATER than 88cm.

I hope this helps.