Hi Marissa.

Here's how you do it:

1. Guess at the price of the sleeping bag: I will start by guessing it is $15.
2. Calculate how much the mess kits cost based on the guess and the first bill. The first bill is $37 and the sleeping bag is $15, so that leaves $22 for the mess kits. That makes each mess kit $11.
3. Check the costs of the two items with the second bill and see if they add up. The second bill is for one mess kit ($11) and 2 sleeping bags ($15+$15), so that adds up to $41 - much less than the actual cost of $56.
4. Go back to step 1 and adjust your guess for the cost of the sleeping bag - try to watch to see whether you should go up or down in your guess.

You should eventually find the price of both items.

Cheers,
Stephen La Rocque.