#include <bits/stdc++.h>
using namespace std;
void make_cycle(long long m, long long& sum_cycle, vector<long long>& arr,
                long long& divider) {
  sum_cycle = 0;
  if (m && 1) {
    arr.resize(11);
    arr[0] = 0;
    divider = m * 10LL;
    for (long long i = 1; i <= 10; i++) {
      arr[i] = (m * i) % 10;
      sum_cycle += arr[i];
    }
  } else {
    arr.resize(6);
    arr[0] = 0;
    divider = m * 5LL;
    for (long long i = 1; i <= 5; i++) {
      arr[i] = (m * i) % 10;
      sum_cycle += arr[i];
    }
  }
}
int main() {
  int t;
  cin >> t;
  while (t--) {
    long long n;
    long long m;
    long long divider;
    cin >> n >> m;
    long long sum_cycle = 0;
    vector<long long> arr;
    make_cycle(m, sum_cycle, arr, divider);
    long long sum_t = (n / divider) * sum_cycle;
    long long remdig = (n % divider) / m;
    for (int i = 1; i <= remdig; i++) {
      sum_t += arr[i];
    }
    cout << sum_t << endl;
  }
  return 0;
}
