Why isn't an increase and decrease symmetrical?
The base changes. The increase is measured against the smaller starting number and the decrease against the larger one, so the same $180 move is a different share of each.
Calculate the percentage increase or decrease between any two numbers, and see why the same absolute move gives different percentages in each direction.
Percentage change = (new − old) ÷ old × 100. Percentage difference = |a − b| ÷ ((a + b) ÷ 2) × 100.
Rent rising from $1,200 to $1,380 is (180 ÷ 1200) × 100 = a 15% increase. Going back from $1,380 to $1,200 is a 13.04% decrease, not 15%.
The base changes. The increase is measured against the smaller starting number and the decrease against the larger one, so the same $180 move is a different share of each.
It compares two values with no 'before' and 'after' by dividing the gap by their average — useful when neither number is the baseline.
Divide by 1 plus the rate. To strip a 15% rise from $1,380: 1380 ÷ 1.15 = $1,200. Subtracting 15% would incorrectly give $1,173.