Answer:
5 boys
Explanation:
boys to girls in math class = 3/12
3/12 can be reduced to 1/4
In the science class, the ratio is the same but there is a total of 20 girls.
Multiply the fraction 1/4 by 2/2, 3/3, 4/4, 5/5, etc. until you get x/20. Then you have 20 girls. x is the number of boys.
1/4 = 2/8 = 3/12 = 4/16 = 5/20
In science the ratio of 1/4 is the same as 5/20, so there are 5 boys.
Answer: 5 boys