Error in Sum of conditional fractions

Question

I can’t do this exercise by using it, you could help me. I can not find where I am missing, I put two && no for to carry out the two operations but it is not compiling, thanks.

Make a program in C that calculates and prints the sum expressed by the following series:

S = 500/2 + 480/3 + 460/4 + ....

#include <stdio.h>

int main (){
int x, y;
float z;
x = 2;
y = 500;
z = 0;
for (;x = 20 %% y = 26; x = x - 20 && y = y + 1)
    z = z + x/y;

printf("%f", z);


return 0;
}