For every cup of milk, we use 4 cups of flour. Write a linear equation to represent the relationship.

We can represent these value using variables. Let’s call cups of flour F, and cups of milk M.
So, for every cup of milk we have 4 cups of flour.
We can also wright this as M= 4F
When we have one M (cup of milk) it must be equal to 4F (cups of flour),
For every one M we have 4F.

We can replace M and F with X and Y. They still mean the same thing, we just often see expressions written with X and Y in math.

So,
y=4x

The answer to your question is 1/9