The strategy
How it works
Calculate the percentage change from the original amount.
Subtract the change for a discount or decrease; add it for an increase.
Check the direction: a discount should lower the price, an increase should raise it.
See it in action
Worked examples
Example 120% off $50
- 10% of $50 is $5, so 20% is $10.
- Subtract the discount: $50 − $10.
- Sale price = $40.
Answer: $40
Example 2Increase 80 by 10%
- 10% of 80 is 8.
- Add the increase: 80 + 8.
- New amount = 88.
Answer: 88
A quick heads-up
Common mistakes to avoid
- Reporting the discount amount as the sale price; subtract the discount from the original.
- Taking a percentage of the already changed amount when the problem specifies a one-time change from the original.
Your turn
Check your understanding
Choose an answer to see instant feedback and a short explanation. You can take the quiz again any time.
Question 1 of 3
A $40 item is 10% off. What is the sale price?
10% of $40 is $4; subtract it from $40 to get $36.
Increase 60 by 5%. What is the new value?
5% of 60 is 3; add the increase: 60 + 3 = 63.
After a 25% discount, what fraction of the original price remains?
100% − 25% = 75%, which is three quarters of the original price.
Keep the momentum
Try a few practice problems
Choose a comfortable range and work through a short round with the free mental math trainer.