This is an old question, but worth some explanation. I had fun reading some of the answers here; the problem statement is obviously a prank!
These types of problems are units of measurement problems and are solved as Daniel did.
The units need to be consistent. While Daniel attempted to keep the quantity and the $$ separate, and got the end result correct, the units got mixed up.
The problem should be written as:
one bushel of grain = 3 sacks of oats + 2 sacks of barley => 7,000 bushels = 3x + 2y (x,y expressed in sacks)
cost of oat = $1.10/sack ; cost of barley = $2.10/sack => $4,275 = 1.10x + 2.10y (x,y expressed in sacks)
The problem statement therefore is:
How many sacks of oat and barley does Mrs Lewis need to buy? (parts got everyone confused - haha)
The solution is solved in sacks, certainly not in bushels. Only in this way can it be solved as Daniel describes it.
If the unit of measurement for oat and barley is bushels, the problem is not solvable, as some had fun pointing out.
A 2 equation problem involves 3 units of measure. in this case:
equation 1 : bushel/sack
equation 2: $/sack
The three units being $, bushels and sacks (or parts)
I hope no student followed this thread - lol