The total, $5{,}080$, is in kilograms of fertilizer. If 12 and 28 are acres, what units must $x$ and $y$ have so that $12x$ and $28y$ also end up in kilograms?

Once you know the units of $y$, ask: does $y$ go with Gold barley or Silver barley in the equation?

The equation is $12x + 28y = 5{,}080$.

• The farmer plants Gold barley on 12 acres.
• The farmer plants Silver barley on 28 acres.

So the 12 in $12x$ matches the 12 acres of Gold barley, and the 28 in $28y$ matches the 28 acres of Silver barley.

The total fertilizer, $5{,}080$, is measured in kilograms.

That means each term on the left side, $12x$ and $28y$, must also be in kilograms:

• $12x$ = fertilizer used on Gold barley, in kilograms
• $28y$ = fertilizer used on Silver barley, in kilograms

Since 12 is in acres, to make $12x$ come out in kilograms, $x$ must be in kilograms per acre. The same logic shows that $y$ must also be in kilograms per acre.

Now connect the variables to the barley types:

• $12x$ uses the 12 acres of Gold barley, so $x$ is the fertilizer needed per acre of Gold barley.
• $28y$ uses the 28 acres of Silver barley, so $y$ is the fertilizer needed per acre of Silver barley.

Therefore, the best interpretation of $y$ is “The kilograms of fertilizer required per acre of Silver barley.”