The length and breadth of a rectangle are increased by 8% and 5%, respectively. By how much percentage will the area of the rectangle increase?

13.4%

We know that,

When the two percentages A and B are given then the final change in the percentage= (A + B + \(\frac{AB}{100}\))%

The length and breadth of a rectangle are increased by 8% and 5%, respectively. 

Final percentage increment in area = (8 + 5 + \(\frac{8 × 5}{100}\))% = 13.4%