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

${\displaystyle X_{n+1}=X+(1-\frac{p}{100})X_n}$

Put $X_n=Y_n-C$ then choose $C$ such that

${\displaystyle Y_{n+1}=(1-\frac{p}{100})Y_n}$

and consider the values of this expression for $Y_n,Y_{n-1},Y_{n-2},...Y_1,Y_0$.