5friend's are having a tennis tournament. each friend will play the other 4 friend once. how many matches will be played

The handshake problem!
Each of the five will play against 4 others, but since A vs B can be considered the same game as B vs A, so there are 5*4/2=10 matches.

This law is sometimes called the grouping property. as you can see, regardless of how the numbers are grouped, the answer is still 13. the associative law of multiplication is similar to the same law for addition. it says that no matter how you group numbers you are multiplying together, you will get the same answer.