Explanation:
Given X total money, A cost of the cake, B cost of Donut.
After Buying a cake we need to buy as many donuts as possible
and output the remaining amount
hence X=A+(kB)+Z
(X-A) = (kB) + Z
where K is maximum donuts bought
and Z is remaining money
which is basically Z= (X-A)%B
#include<bits/stdc++.h>
using namespace std;
int main()
{
int x,a,b;
cin>>x>>a>>b;
cout<<(x-a)%b<<'\n';
}