Let g = graph paper and r = ruler
1st equation: 5g + 30r = 40
2nd equation: 12g + 20r = 44
Multiply the 2nd equation by -1.5 to cancel out the r's
2nd equation: 12g + 20r = 44 x -1.5 = -18g - 30r = -66
Now add the 1st equation to that:
-18g + 5g = -13g
30r -30r = 0
40 -66 = -26
The new equation becomes -13g = -26
Divide both sides by -13
g = -26 / -13
g = 2
This means each pack of graph paper cost $2.
Now replace g with 2 in the first equation to solve for the cost of a rule:
5(2) + 30r = 40
Simplify:
10 + 30r = 40
Subtract 10 from each side:
30r = 30
Divide both sides by 30:
r = 30 / 30
r = 1
Each rule cost $1.