Question

Jonathan's class has 30 boys of the students in his class 60% are girls how many girls are in Jonathan's class

Answer 1

we can infer that 30 : 40% (because the rest are girls)
and then X = 60%
by the criss cross and elimination method
40 X = 30 * 60
thus There are 45 girls

Answer 2

Something's not quite right here. Sounds as though 60% of the 30 boys are girls. ;)

Suppose that there are n students in Jon's class, of which 30 are boys.
We are told that 60% of those n students are girls. That means that the remaining students (40%) are boys.
30 boys
0.40n = 30 boys, so n = ------------ = 75. There are 75 students in the class.
0.4

60% of these 75 students are girls: 0.60(75 students) = 45 girls