Taking X_n as the amount of stock at the end of week n, you need to solve the difference equation (recurrence relation)

Xn+1 = X + (1 - p 100 )Xn

Put X_n = Y_n - C then choose C such that

Yn+1 = (1 - p 100 )Yn

and consider the values of this expression for Y_n, Y_n-1, Y_n-2, ... Y₁, Y₀.