Start After +20% Then −20%
Start
$100
After +20%
$120
Then −20%
$96
Add the percent, then take the same percent off — you don't land back on the start. The decrease is taken off a bigger number, so it's a bigger cut.
10% of 50 10% of 80
Same percent, different wholes → different amounts. "10% off" only means something once you know 10% of what.
Write 35% as a decimal:
Per-cent = "out of 100". To go from % to a decimal, divide by 100 (slide the point two places left). Going back, × 100. The classic slip is dividing by 10.