If they're going to use 30 sponges, I think the costs would be as follows:
A - pack of 5 @ $6 total, implies 30 sponges at $36 (6 packs needed)
B - pack of 10 @8 total, implies 30 sponges at $24 (3 pakcs needed)
C - pack of 15 @ $10 total, implies 30 sponges at $20 (2 packs needed)
So initially, it would seem that C is the best deal. But if using C sponges would require 60 sponges instead of 30, then the cost of using C-type sponges would double to $40 (since you'd need 4 packs instead of 2), making B the best deal at $24 in total.
Does that sound right?
- Steve J