View Code of Problem 3920

#include<iostream>
using namespace std;
int main() {
	 int arr[10][13] = {}; arr[0][12] = 1;
	for (int i = 1; i <= 9; i++) {
		arr[i][0]=0;
		int flag[10] = {};
		for (int j = 1;!flag[(i*j)%10]; j++) {
			int v = (i*j) % 10;
			flag[v] = 1;
			arr[i][j]=v;
			arr[i][0] += v;
			arr[i][12]++;
		}
	}
	int T; cin >> T;
	while (T--) {
		long long n, m; cin >> n >> m;
		long long sum= n / m / arr[m % 10][12] * arr[m % 10][0];
		for (int i = 1; i <= n / m % arr[m % 10][12]; i++)
			sum += arr[m % 10][i];
		cout << sum << endl;
	}
}