An airline has 15 flights from city A to city B and eight flights from city B to city C. In how many ways can you fly from city A to city B.
that is one type of problem another type is Karen has Five rabbits: 2 black, 2 white, 1 black and white. In how many ways can Karen select two of her rabits and include at least one black rabbit.
Two problems are solved using formulas for combinations and permutations.