Reverse Percentage Lookup

问题

This must be a simple question but I can't work it out!

I have a number that I know has been reduced to 70% of its original size, how do I find the original number?

For example I have the number 9.1, which is 13 x 70%, but what is the calculation for finding 13?

回答1:

You want

originalNumber = reducedNumber / (percentage / 100);

for example

13 = 9.1 / (70 / 100)

回答2:

Well if you have some number x, you're looking for a y so that

 y * 70% = x
 y * 70/100 = x
 y = 100 * x / 70

回答3:

Taking your example. Say you have an item with DiscPrice = 9.1 and the discount amount is Disc = 30% and you wanted to calculate the original `SellingPrice' before the discount. Then the equation will be:

SellingPrice = ( DiscPrice * 100 ) / ( 100 - Disc )

// in your case then it will be
SellingPrice = ( 9.1 * 100 ) / ( 100 - 30 )
             = 13

See it in action:

The above formulate for the decrease percentage calculation. There is also an increase percentage calculation here: https://stackoverflow.com/a/54125117/850840

来源：https://stackoverflow.com/questions/20405793/reverse-percentage-lookup