Question 839594
Total weight of class = 150*30 = 4500
Set b = number of boys
 g = number of girls
40b + 25g = 4500 , 40kg x number of boys + 25kg x number of girs
also
30b + 30g = 4500
Since each left side = 4500,
30b + 30g = 40b + 25g
add -30b to each side
30g = 10b + 25g
add -25g to each side
5g = 10b
g = 2b
substitute 2b for g in 30b + 30g = 4500
30b +30(2b) = 4500
30b + 60b = 4500
90b = 4500
divide each side by 90
b = 50
since g = 2b,
g = 100