A percentage increase or decrease is always measured against the amount you started from. To find the percentage change, work out the change first, then compare it with the ORIGINAL amount and multiply by 100. To apply a change, an increase of 15% makes the new amount 115% of the old one and a decrease of 15% leaves 85% of it. When a price changes twice, the second percentage is of the amount after the first change, not of the original — which is why a 20% rise followed by a 25% fall is not a 5% fall.
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