C :

#include<stdio.h>
 
#define maxTime 21*7
 
int main() {
    int a[maxTime] = {0};
    int sum = 0;
    int i ;
    for (i = 0; i < 21; ++i) {
        a[sum] = 1;
        sum += 5;
    }
    sum = 0;
    for (i= 0; i < 21; ++i) {
        a[sum] = 1;
        sum += 6;
    }
    sum = 0;
    for (i = 0; i < 21; ++i) {
        a[sum] = 1;
        sum += 7;
    }
    int total = 0;
    for (i = 0; i < 21 * 7; ++i) {
        total += a[i];
    }
    printf("n=%d", total);
 
}