Please how do i solve this discount payback period eg 100000000 as cash flow and 25% as discount rate

A/(1+r)^n

Where
n is the year
A is the amount
R is the rate

Use the discounting formular to calc for each year.
Note: A is not constant, you keep replacing it with the discounted amount from prior computations until you get to your desired stappage year. Same as n-if you are computing for the 5th year for instance, n will be 5