#include <bits/stdc++.h>   using namespace std;   const int mara = (int)(1e6) + 5;   vector<long long> a(mara), prime(mara), v(mara), luck;   map<long long, bool> flag;   long long bigmod(long long a, long long b, long long m) {     long long res = 1;     while (b) {       if (b & 1) {         res = (res * a) % m;         b--;       } else {         b /= 2;         a = (a * a) % m;       }     }     return res;   }   void soln() {     long long a, b, sum = 0;     cin >> a >> b;     for (long long i = 1; i <= 10; i++) {       long long as = i * b;       as %= 10;       sum += as;     }     long long vajok = 10 * b;     long long div = a / vajok;     long long rem = a % vajok;     long long res = sum * div;     for (long long i = b; i <= rem; i += b) {       long long temp = i;       temp %= 10;       res += temp;     }     cout << res << endl;   }   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     ;     long long t;     cin >> t;     while (t--) {       soln();     }     return 0;   }   