b ... the number of boys
g ... the number of girls
80 children
three times as many boys as girls ... 3 * g = b
If you would like to know how many boys and girls are in a group of 80 children, you can calculate this using the following step:
b + g = 80
3 * g + g = 80
4 * g = 80 /4
g = 80/4
g = 20 girls
b = 3 * g = 3 * 20 = 60 boys
Result: There are 60 boys and 20 girls.