The length of a rectangle is increased by 25% while its breadth is decreased by 20%. What is the effect on its area?

The new area is $1.25\times 0.80=1.00$ times the old area, so it does not change at all. By the successive-change rule, $25-20-(25\times 20)/100=5-5=0$. The usual slip is to stop at 25 - 20 and report a 5% increase (or, with the signs muddled, a 5% decrease); adding the changes as 25 + 20 gives 45%.