Alright, lets get started.
Suppose total number of dancers in school = x
We have given in question, 42 % of total dancers are ballet dancers.
So, we could find number of ballet dancers by finding 42 % of total dancers.
Number of ballet dancers = x * 42 %
Number of ballet dancers = x * 42/100 = 0.42 x
As per given in question, there are 63 ballet dancers in school.
Which means 0.42 x = 63
Dividing 0.42 from both sides
0.42 x / 0.42 = 63 / 0.42
x = 150
Hence number of total dancers in school = 150 : Answer
Hope it will help :)