Sara is going to the store to buy some groceries. The price of 2 gallons of milk is $8 and the price of 6 boxes of Lucky Charms cereal $36. Sara ends up spending $90. Which equation represents the relationship between the number of gallons of milk, x, and the number of boxes of Lucky Charms, y, that Sara bought at the grocery store?

So the first thing we do is to multiply the top equation by 3. the reason i want to do this is i notice there will be a 6y on both equations and can possibly add/subtract them. so we get 6y = -3x + 27. adding that and the second equation we get 6x = 27-15 which is 12. divide by 6 and we get 2. ∴ x = 2. substituting 2 into the first equation we get 2y = -2 + 9 which is 7. divide by 2 and we get 7/2. ∴ y = 7/2, x = 2.