Solution. We will use A to mean paying by cash and B to mean paying with a credit card

We will use A to mean paying by cash and B to mean paying with a credit card. The tree diagram is shown below.

The probabilities of his method of payment are written on the branches.

By multiplying the probabilities along one branch we find the probability of one outcome. Hence in this situation there are four possible outcomes. If we add all the probabilities of the outcomes together, the answer will be 1.

a) In this case we need the branch marked *.

P(AA) means paying by cash on the first visit and on the second visit.

b) In this case we need the branch marked .