A percentage is a fraction of one hundred. Fifteen percent of 200 is 15 ÷ 100 × 200, which is 30. The share reverses that: 30 is 15% of 200 because 30 ÷ 200 × 100 = 15.
Change = (ending − starting) ÷ starting × 100
From 80 to 100, the change is 25%. Adding 10% to 200 gives 220. Subtracting 10% from 200 gives 180. Taking 10% off after adding 10% does not return to the original amount, because the second step uses a different base.