Find the percentage change from an original value and a new value

Follow this one with me, a step at a time. Try to guess each step first.

Worked example 1

The price of a school bag changed from $380 to $410. Find the percentage change, in %, correct to 1 decimal place. A decrease is a negative change.

Remember: Percentage change compares the change with the original value, the value before anything happened.

The change is 410 − 380 = 30.
The original value is 380, so the percentage change is 30 ÷ 380 × 100%.
This is about 7.9%, so the answer is 7.9.

Common mistakes

Gave the new value as a percentage of the original value. That still includes the original 100%.
Subtracted the wrong way round, so an increase became a decrease or the reverse. Take the new value minus the original value.
Divided by the new value. Percentage change is always measured against the original value.