50-60 points are 0.005* 10* 100=5.

There are 0.02* 10* 100=20 families with 60-70 points.

There are 0.04* 10* 100=40 families with 70-80 points.

There are 1000a families with 80-90 points.

Families at 90- 1000 have1000 b.

1000 (A+B) = 35， 1000 (A-B) * 8/ 100 = 3，

So a+b=0.035, a-b=0.0375,

The solution is b =-0.00 125.