we can write b= boys, and g= girls
we know that girls are equal g=(2*b-5)
we also know that b+g=40
we will replace girls with the above equation and get
b+ (2*b-5) = 40 , 2*b=2b
b+2b= 40+5
3b=45, divide everything by 5
b=15
girls are 2*15-5=25
15+25=40