The fast method
⚡
10 + 20 - (10 × 20 / 100) = 28.
How it works
For discounts a% and b%, the effective discount is a + b - ab/100 percent. Remaining price fraction is (1-a/100)(1-b/100). Subtracting from 1 gives the formula.
Why it works
Remaining price fraction is (1-a/100)(1-b/100). Subtracting from 1 gives the formula.
Use it when
Both discounts must be applied successively to the reduced price, with rates from 0 to 100.
Watch out for
Do not simply add the discount rates.
Verified examples
Example 1
Combine 10% and 20% discounts
10 + 20 - (10 × 20 / 100) = 28.
28%Example 2
Combine 20% and 25% discounts
20 + 25 - 5 = 40.
40%Example 3
Combine 10% and 10% discounts
10 + 10 - 1 = 19.
19%