I have to assume that someone goes into the store and buys 3 pens for $1.50, 8 for $4 and 12 for $6. This is a bit confusing until you verify that the unit cost per pen is the same in all three cases.
3 pens for $1.50 comes out to $0.50 per pen;
8 pens for $4 comes out to $0.50 per pen; and
12 pens for $6 comes out to $0.50 per pen.
Summarizing: c = ($0.50)p
