If the length and breadth of a rectangle are changed by +20% and by -10% respectively, then what shall be the percentage change in the area of the rectangle ?

The area increases by 8%.

New area factor = (1 + 0.20) x (1 - 0.10).

This equals 1.20 x 0.90 = 1.08.

A factor of 1.08 means the area becomes 108% of the original.

The change = 108% - 100% = 8% increase.