You have two options if you have to subtract percentages as part of a math problem. With the first method, you work out the percentage value and subtract it from the original value. With the second method, you work out the remaining percentage and then calculate the percentage value. Both methods of subtracting percentages are simple, so just choose the one you prefer.
Calculate, Then Subtract
Find the Decimal
Change the percentage to a decimal. For example, say you have a math problem asking you to work out a sale price of an item. The item's original price was $27.90 and it has 30 percent off in the sale. This means you want to subtract 30 percent from $27.90. Divide the percentage number by 100 to convert to a decimal. In this case, work out 30 ÷ 100 = 0.3.
Find the Percentage Value
Multiply the original value by the decimal to determine the percentage value. In this example, work out 27.90 x 0.3 = 8.37.
Subtract the Percentage Value
Subtract the percentage value from the original price. In this example, work out 27.90 - 8.37 = 19.53. The new price is $19.53.
Subtract, Then Calculate
Subtract the Percentage
Take the percentage from 100 to find the remaining percentage. In the previous example, work out 100 - 30 = 70. The remaining percentage is 70 percent.
Find the Decimal
Divide the remaining percentage number by 100 to convert to a decimal. In this example, work out 70 ÷ 100 = 0.7.
Multiply by the Decimal
Multiply the original value by the remaining percentage decimal. In this example, work out 27.90 x 0.7 = 19.53. The new price is $19.53.
TL;DR (Too Long; Didn't Read)
If you have a calculator with a percentage (%) key, you can subtract percentages. To subtract 30 percent from $27.90 as per the previous example, type 27.90 - 30 % = to get the answer of 19.53.