Hello Mahonroy(a) is the number of ways of choosing items from a group containing items.

Now we may imagine the items to be divided arbitrarily into two groups of items each, and we may choose:

- items from the first group (which can be done in ways); or

- items from the second group (again in ways); or

- item from each group (which can be done in ways).

Thus the total number of ways of carrying out one of these three options is

Thus

(b)

Grandad