I need a formula for this, if a product is on sale for 3600 dollars and the regular price is 4000 dollars. What formula do I use to get the percentage of what was taken off. Thanks

This may not be the only way to figure this out but here goes nothing

Take 3600 divide 4000 = .9 x 100 = 90% so 100% minus 90% = 10%

The way I learned it is: X % of D is N. X%/100 = N/D