Permutation & Combination

The sum of all four-digit numbers that can be formed with the distinct non-zero digits $$ a, b, c, $$ and $$ d $$, with ...

P, Q, R and S are four towns. One can travel between P and Q along 3 direct paths, between Q and S along 4 direct paths,...

The number of ways of distributing 20 identical balloons among 4 children such that each child gets some balloons but n...