The sides of a rectangle are in the ratio 5: 4. If its perimeter is 72 cm, then its length is_______.

Consider the sides of the rectangle as 5x and 4x. 

We know that, the perimeter of a rectangle = 2 (Length + Breadth)

By substituting the values

72 = 2 (5x + 4x)

On further calculation

72 = 2 × 9x

So we get

72 = 18x

By division

x = $\frac{72}{18}=4$

Hence, the length of the rectangle = 5x = 5 × 4 = 20 cm