Analysis: Before taking out 12 boxes of chalk, the ratio of the number of chalk boxes in box A and box B is 5: 1, which means that the number of chalk boxes in box A accounts for five-sixths of the total number of boxes, and box B accounts for one-sixth; After taking out, the quantity ratio of two boxes of chalk is 7: 5.

Set: two boxes of chalk ***X boxes.

See the figure below for the equation.

Calculated x is 48.

the second question

Suppose the number of boys is 1, then the number of girls is two-thirds and the class size is five-thirds. Then the proportion of girls in the class is two-thirds to three-fifths, which is simplified to 2:5.