Half of the students do math, one quarter draw pictures, one seventh read little English and one seventh play on the playground. How many people have a process?

Therefore, the minimum number of personnel =3/(3/28)=28 (people); Maximum value =6/(3/28)=56 (person)

2、

3x-5y=2a……( 1)

2x+7y=a- 18……(2)

Substituting x=-y into (1)(2) gives: x = a/4 = (18-a)/5; y=-a/4=(a- 18)/5

Get: 5a=4( 18-a)，get: a=8。

Then: x=a/4=2, y=-x=-2.