View Code of Problem 3920

#include<stdio.h>
#include<string.h>
int main(){
	int q;
	scanf("%d",&q);
	while(q--){
		long long int n,m;
		scanf("%lld %lld",&n,&m);
		int i; 
		long long int sum = 0;
		for(i = 1;i <= n; i+=m){
			sum += (i%10);
		}
		printf("%lld\n",sum);
	}
	return 0;
}