View Code of Problem 3920

#include<stdio.h>
#include<string.h>
int main(){      
	int q;
	scanf("%d",&q);
	while(q--){
		long long m,n,i,j,k;
		scanf("%lld %lld",&n,&m);        
		int a[10];
		a[0] = m%10;
		long long int num = m*2;
		for(i = 1;i < 10; i++){
			a[i] = a[i-1] + num%10;
			num = num+m;
		}
//		for(i = 0;i < 10; i++){
//			printf(" %d ",a[i]);
//		}
		i = n/m;              //共有多少个页数可加 
		k = i/10;             //共有多少个a[10 ]
		j =  i%10;             //a[10]外有多少剩余 
		if(j!=0)
			printf("%lld\n",k*a[9]+a[j-1]);
		else
			printf("%lld\n",k*a[9]);
	}
	return 0;
}