how solving this challenge? why is the output 65?

Question

#define x 10 + 5
int main() {
int a = x * x;
printf("%d", a);
return 0;
}

Answer 1

x is defined as 10+5 pre-processor directive. It is replaced as it in program when ever it finds x instance,
So
a= x * x
// so expresions becomes like this
=> 10+5 * 10+5
=> 10+ 50 + 5
=> 65