Matt can buy key chains in packs of 10 for $20 or in packs of 4 for $12. How much money can he save if he chooses to buy 20 key chains at a lower price? Think carefully first, then make a decision:
The cost of each key chain in packs of 10 for $20 is $20 / 10 = $2. The cost of each key chain in packs of 4 for $12 is $12 / 4 = $3. There is a price difference of $3 - $2 = $1 per key chain between the two choices. By buying 20 key chains at a lower price, Matt can save 20x$1 = $20. So, the answer is 20.