at an office supply store emilio bought 3 notebooks and 5 pens for $13.75 if a notebook costs $1.25 more than a pen how does one
notebook cost?
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We have to set up 2 different equations if we are to solve for 2 unknowns. The first equation is x = y + 4. One number (x) is (=) 4 more than another (y + 4). Since we have determined that x is larger (cuz it's 4 more than y), when we set up their difference, we are going to subtract y from x cuz x is bigger. The second equation then is 
. In our first equation we said that x = y + 4, so let's sub that value in for x in the second equation: 
. Expand that binomial to get 
. Of course the y squared terms cancel each other out leaving us with 8y + 16 = 64. Solving for y we get that y = 6. Subbing 6 in for y in our first equation, x = 6 + 4 tells us that x = 10. Yay!