Well, if 30% of students are in the play then 100%-30% = 70% of students are not in the play, which is also 140.
So we know:
70%x=140
70% is also 7/70:
\(\frac{7}{10} x=140\)
divide both sides by 7 and multiply by 10:
x=2010
x=200
there are 200 students in the school