February 2013 Brain Teaser Solution

You must buy 100 pens for exactly $100 and purchase at least one pen from each of three stores. The first store charges 5 cents per pen, the second charges $1 per pen and the third charges $5 per pen. How many pens should you buy from each store?


Well I know it has to be an exact dollar amount. So the first store needs to be 20, 40 or 60 pens. I know the first 2 stores have to be a multipler of 5 for $ and the third store needs to be between 5 and 19 pens. And the second and third store has to be a multipler of 20 for pens.

So

80 Pens
1 Pen
19 Pens