When is it helpful to take a certain percentage off a number? This can happen when you see that a store allows discounts, for example on Black Friday.
Here you can calculate how much the price is after applying the discount. You can calculate the sale price of an item on sale and how much money you will save.
Reducing a number by a percentage can also be instrumental when you learn that the gas price will fall.
A company can also reduce production by a certain percentage and you want to know which difference in produced items this represents.
If you want to subtract P percent off a value Y, you can use this formula:
result = Y - (P/100*Y)
For example subtract 5 percent of 120: result = 120 - (5/100*120) = 114