In a class there are 27 students. knowing that the number of girls represents 80% of the number of boys, how many girls are in the class

G = 0.8b         where g = number of girls and b = number of boys.

also g + b = 27 
substituting for g:-

1.8b = 27
b = 27/1.8 = 15

so number of girls g = 0.8*15 = 12 answer

There should only be 1 solution