Set up two ratios: 3:4 and x:48. The ratio always stays the same, so the value that you increased the girls by will be equal with the boys. To get from 4 to 48, you need to multiply by 12. So, to get a proportional number of boys, multiply 3 x 12.
The ratio of boys to girls is 36:48. The question is asking how man total, so add. 36 + 48 = 84.
There are 84 total sixth graders.
