Suppose that 70 people in the class are all boys, then 70/4= 17+ 1/2 people will take part in the activity.

2.5 people are missing.

If one of these 70 people becomes a girl, the number of people participating in the activity will decrease by 1/4 and increase by 1/3.

So when a boy becomes a girl, the number of participants increases1/3-1/4 =112.

So it takes 2.5/(112) = 30 boys to become girls to make up for the missing 2.5.

So the answer is 30 girls and 40 boys.

Formula:

(70/4) ÷ (1/3-1/4) = 30 girls.

70-30=40 boys