The graph tells us that as you buy more pens, the price increases. Let's look at the prices and numbers of pens bought.
Bought 6 pens for $7.50
Bought 3 pens for $3.75
Between both of those points we either doubled (going from 3 to 6) or halved (going from 6 to 3) and the cost doubled or halved the same way. We can make a proportion for this relationship.
Let C be the cost of 1 pen. On the left side is information for 3 pens, on the right side is information for 1 pen, on the top is how many bought, on the bottom is cost.
[tex] \frac{3}{3.75} =\frac{1}{C} [/tex]
We solve by cross multiplying.
3C = 3.75
C = 1.25 after dividing both sides by 3.
Thus each pen costs $1.25 and the third choice is best.
