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30% of all students are in a play at school. All students except for 140 are in the play. How many students are in the school?

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The key to this one is figuring out what percent of the students are NOT in the play.  If 30% of the students are in the play, that means that 70% are not, right?  And we know that 140 students are not in the play.  So 140 people is 70% of the total students, which is what we want to find out.
For a problem like this, I always set it up as a proportion.  Remember that 100% is the whole thing.
140/70 = x/100    (partial number/partial percent = whole number/whole percent)
2 = x/100
x = 200
So there are 200 students in the school.