#include <bits/stdc++.h>   using namespace std;   long long a[150500];   int main() {     ios_base::sync_with_stdio(false);     int t;     cin >> t;     vector<long long> ans;     for (int f = 0; f < t; f++) {       long long n, m;       cin >> n >> m;       vector<long long> l;       l.push_back(m % 10);       for (long long i = m + m; l[0] != i % 10; i += m) l.push_back(i % 10);       long long c = n / m / l.size() * accumulate(l.begin(), l.end(), 0);       for (int i = 0; i < n / m % l.size(); i++) c += l[i];       ans.push_back(c);     }     for (auto i : ans) cout << i << endl;     return 0;   }   