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

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

Answer 2

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