I'm so confused right now. I don't need the answer, I just don't know how to put it into equation form.

Let x be the number of students in the senior class.

Four-fifths of this number x is equal to 324 based on the first sentence and the beginning of the second sentence.

So the equation to set up is and it's the same as writing

If you want, you can stop reading here and attempt to solve on your own.

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On the other hand, if you want to know how to solve, then read on.

The '5' in the denominator means we're dividing 4x over 5.

To get that 5 to the other side, we'll multiply both sides by 5 to undo the division operation.

Then to fully isolate x, i.e. get x all by itself, then we'll divide both sides by 4. This is to undo the multiplication of 4 onto x.

So,

This means there are 405 seniors overall.

4/5 of them went to prom, so (4/5)*405 = 324 went to prom. This confirms the answer.