Company X provides bottled water to its employees in 5 liter bottles, $2 each. At least how many 5L bottles must Company X buy monthly, so that new contract with $50 fixed monthly rate and $1 for 20L bottle each paid off? (Assume that no other costs apply)
Number of 5L bottles = n
Correspondingly, to equate volume, number of 20L bottles = n/4
Now equating cost to solve:
2n = 50 + n/4
=> 8n = 200 + n
=> 7n = 200
=> n = 28.57
So 29 bottles, since a fraction of a bottle is no use and 28 bottles is going to result in a loss.
It is great you came up with an original problem (and that too based on your business), but I'd like to slightly rephrase in this question if I could: "...must Company X
You've mentioned no additional costs apply other than the ones you've mentioned, but the solution still has to assume the 5L bottles will be bought at $0 cost, which might throw some folks off.
Still, kudos for creating your own question. I'd agree with you that this is a 600-700 level question. Now if you introduced red and blue bottles and rate of pouring, it'd become a 700+ problem.
The only ability the GMAT is an indicator of...is the ability to do well on the GMAT.