A group of 13 friends were planning a trip. On night before they made lot of phone calls. Each friend talked to every other friend atleast once. what is fewest phone calls that can be made?

13 * 12 = ?

thank you Ms. Sue. But how did you arrive to 13*12? can you please explain - i did not understand

Each of the 13 friends called 12 other people.

To find the fewest number of phone calls that can be made, we can use the formula for the number of combinations of choosing 2 elements from a set of n elements, which can be calculated as n(n-1) / 2.

In this case, there are 13 friends in the group. So, we can calculate the number of phone calls using the formula:

13 * (13-1) / 2 = 13 * 12 / 2 = 78

Therefore, the fewest number of phone calls that can be made is 78.