Pick any amount you like for the initial size. (The actual amounts are not the point: you're looking at fractions and proportions.)

Find 20% of this. Subtract it from your initial amount.

By what amount is the new, smaller amount lower than the initial amount? What percentage is this shortage of the new amount?

This is the percentage of the new amount by which he would have to increase the new amount, in order to return to the original amount.

(Hint: Your answer should be somethinglargerthan 20%.)