.
The number of those who plays at least one sport is 21 - 11 = 10.
Therefore, the number of those who plays both sports is 7 + 8 - 10 = 5.
The probability under the problem's question is P = . ANSWER
Solved.
----------------
To understand the solution in FULL, read and learn from the lesson
- Counting elements in sub-sets of a given finite set
in this site.
/\/\/\/\/\/\/\/
If you want communicate with me (and have a two-ways communication), always refer in your comment/message
to the problem's ID number.
For this problem, its ID number is 1161210.
Find it in your file, in the VERY FIRST upper line.
Then you will obtain my response in the same post/spot.
If you do not refer to the problem's ID, I will not know to whom to answer.
It is how this forum works.