B = number of boys
G = number of girls
G = 3*B since there are 3 times as many girls compared to boys
B+G = 60
B+3B = 60 ... replace G with 3B, solve for B
4B = 60
4B/4 = 60/4
B = 15, so there are 15 boys
G = 3*B
G = 3*15 ... plug in value of B
G = 45, and 45 girls
as a check
B+G = 15+45 = 60, so that works out
