The macro is substituted like the following ,

Code:
#include<stdio.h>
#define SQR(x) x * x
int main()
{

    printf("%d", 225/15*15);

}
Because of the operator precedence 225/15 executed first and have the result 17 .Then the 17 multiplied by 15 .So the result is 225.So use parenthesis .