Abaker used 4 cups of flour to make 5 batches of brownies. he used_of a cup of flour to make 1 batch of brownies

(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 :)