Half are studying math, one quarter are studying music, one seventh are studying foreign languages, and the rest are:

1- 1/2- 1/4- 1/7=3/28

There are less than six students playing football on the playground, so the number of students playing football accounts for 3/28 of the total number. The number of students playing football can only be 1, 2,3,4,5, and the number must be an integer, so only three students meet the requirements, and the total number is:

3/(3/28)=28