1. Percentage Changes

2. Percentage Changes
increase = new value – original value
decrease = original value – new value
Percentage increase and percentage decrease are collectively called percentage change.

3. +20% means that the original value is increased by 20%.
When new value > original value, a positive percentage change is obtained (i.e. a percentage increase).
e.g. When 100 is changed to 120,
new value = 120,
original value = 100
+20% means that the original value is increased by 20%.

4. –20% means that the original value is decreased by 20%.
If new value < original value, a negative percentage change is obtained (i.e. a percentage decrease).
e.g. When 150 is changed to 120,
new value = 120,
original value = 150
–20% means that the original value is decreased by 20%.

5. We can change the subject of the formula to ‘new value’:

6. Follow-up question
1. Mary’s salary was $ last year. If her salary is $ this year, find the percentage change in her salary.
Solution
Percentage change in her salary × – =
100%
600 14
184 15 % +4 =

7. Follow-up question (cont’d)
2. Tom wants to reduce his expenditure for next month by 10%. If Tom’s expenditure this month is $16 200, find his expected expenditure for next month.
Solution
His expected expenditure
10%) (1
200
$16 - × =
580
$14 =