A large sack of sand is $3.25 for 50 pounds; a small sack is $2.29 for 30 pounds. You need 170 pounds of sand. What’s the cheapest way to buy it?

The cheapest way to buy 170 lb of sand is actually to buy 180 lb in three 50-lb sacks and one 30-lb sack:

3 x 50 + 30 costs 3 x 3.25 + 2.29 = $12.04.

You can always find something to do with an extra ten pounds of sand.