Three people exchange phone calls once, one * * * makes () phone calls, four people send a text message to each other, and one * * * sends () text messages.

A * * * called three times; , a * * * sends 12 SMS.

Here we need to consider the influence of the arrangement order on the results:

1, three people communicate with each other once, and each person calls others twice, but the result of A-to-B call and B-to-A call is the same, so the number of calls of a * * * is (2+2+2)÷2=3 times;

2. Four people send messages to each other, and each person sends messages to others three times. There is no order here. A short message sent by Party A to Party B is different from that sent by Party B to Party A, so Party A * * * has to send 3+3+3 = 12 short messages.

Extended data:

Basic counting principle and application of two commonly used arrays

1, addition principle sum classification counting method;

Each method in each class can accomplish this task independently; The specific methods in the two different methods are different from each other (that is, the classification is not heavy); Any method to accomplish this task belongs to a certain category (that is, classification does not leak).

2. Multiplication principle and step counting method:

One method of any step can't complete this task, and only by continuously completing these n steps can this task be completed; Each step is independent of each other; As long as the methods used in a step are different, the corresponding methods to complete it are also different.