(I dont know if this is the way your teacher told you how solve the problem but I'm gonna try it anyway)
First of all, Let x = the mystery amount of flour
To create my equation, I put cups of flour over the amount batches in a formation, kind of like a fraction.
cups
-----------
batches
Next, I fill in the values.
cups 4
----------- = ---------
batches 5
The 4/5 represents the first ratio of flour/batches.
cups 4 x
----------- = --------- = ---------
batches 5 1
Since we do not know that amount of flour it took to make one batch, we put x in it's place.
Now it's time to cross multiply. It's exactly like it sounds! Multiply diagonally.
Multiply 5 and 'x'. Multiply 4 and 1.
You end up with a eqaution that looks like:
5x = 4
As you would for any solve-by-x equation, divide the five on both sides.
5x= 4
- -
5 5
4/5 = Four-Fifths
5/5 cancels itself out and poofs into thin air.
You end with x = 4/5
This is the answer.
The baker used 4/5 cups of flour to make 1 batch of brownies :)