Answer:
12 boys
Explanation:
60% is the same as 6/10, and if you make the denominator 30, you must make the denominator multiply by three as well as the numerator. this makes the number 18/30 girls in a class. if you subtract the girls from the total number of people, you get 12 boys, or 40% of the class.
you can also find that 100% - 60% = 40%, which is equal is equal to 4/10. After it's multiplied by 3 again, it becomes 12/30 or 12 boys.