Percent Decrease

A percent decrease works exactly like an increase, from the other side. A town had 50 trees and now has 40. The change is the drop, 50 - 40, which is 10 trees. Measured against the original 50: (10/50) × 100 is 20. The number of trees fell 20%.

On the board, write the bigger, original number first so the change comes out positive: p = ((50 - 40)/50) × 100, then p = 20. The word decrease in the question already says which way it went, so p is just the size of the drop.

Notice that going from 40 back up to 50 is a 25% increase, not 20%. The change is 10 both times, but the starting point differs. That is why the original always goes underneath.