The third edition of discrete mathematics answers after class.

It is simple to solve madness with the principle of exclusion.

Let the participants in the football match be set as A;

Set the people who participate in the basketball game as set B;

Set the participants in the volleyball match as set C;

Yes: (because exchange is not fun, subtraction for exchange and addition for alliance).

|A|=28，|B|=29，|C|=26，|A-B|=7，|B-C|=9，| A-C | = 1 1；

Know the principles of addition and exclusion:

| A+B+C | = | A |+| B |+| C |-| A-B |-| A-C |-| B-C |+| A-B-C |

Namely:

60 = 28+29+26-7-9- 1 1+| A-B-C |

Get:

|A-B-C|=4

So the answer is 4.