Atheater can hold 140 giants or 210 elves. if 90 giants are already inside, how many elves can also be admitted?

75 elves.

I created a fraction to help solve for the number of elves. I did so by:

90/x=140/210 

I solved for x which would equal 135. This is the equivalent number of elves in the theater. You would subtract this number from 210 which would equal 75.

OR 

You can figure out how many elves equal 1 giant. In this case, you would divide 210 by 140 which equals 1.5 giants. Since there are 90 giants, you multiply 1.5 giants to find out an equivalent for elves which would equal 135. Same as before, subtract this from 210, resulting in 75.