At a back to school sale Chris bought 3 pencils and 7 pens for $3.55. At the same sale his brother James bought 7 pencils and 3 pens for $2.95. How much would it cost to purchase 1 pencil and 1 pen?

Let a represent pencils and b represent pens. Then we can create the equations: 3a + 7b = 3,55 and 7a + 3b = 2,95. Isolate the a in the first equation: a = (3,55-7b) / 3 and put the isolated a into the second equation: 7 (3,55-7b) / 3 + 3b = 2,95 = > 7*3,55/3 - 7*7b/3 + 3b = 2,95 = > 7*3,55/3 - 2,95 = 49b/3 = > b = 5,333/16,333. To find a put the value of b into the first equation: 3a + 7*5,333/16,333 = 3,55 = > a = 0,42146 and b=5,333/16,333 = 0,32652