percentage - How to get the actual value after subtracting discount(%) -

i want total amount of final amount , discount rate.

for example.

grand total = 100

discount = 15%

final amount =100*.15=85

if have 2 variables

final amount = 85

discount = 15%

grand total = ?

now can grand total of "final amount" , "discount rate"?

let grand total = x

x-(x * 15 %) = 85 if find value of x grand total can e found.

x - 3x/20 = 85

17x= 85*20

x = (85/17)*20

x =100

put .. find grand total