In 2005, the US Mint in Denver produced 4.04 × 108 Oregon State quarters. The US
Mint in Philadelphia produced 7.12 × 107
Oregon State quarters. Considering the
other two US Mint locations did not
produce any quarters, how many Oregon
State Quarters were minted in 2005?
Express your answer in scientific notation.

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.