What's In the Box?

Congratulations, you solved the math problems and freed the baby polecat! Your captors are happy too.  They let you know that they are not from V.A.L.A., but refuse to tell you who they are with, saying only, "By the time we're done with you, you'll wish you were captured by those idiots V.A.L.A.!"

So far you have only had moldy bread to eat and fetid swamp water to drink. You demand something better to eat, but they just laughed at you and gave you a mysterious box with a combination lock on it.

They said, "Solve this next set of problems if you want to find out what's in the box."

Problem 12, eq (1):

Problem 12, eq (2):

Use J and A for variables in your answers

Problem 12, How much did Jon spend?

Problem 12, How much did Amy spend?

Problem 13, width in m:

Problem 13, length in m:

Problem 16:

Problem 17:

Problem 18:

Problem 19, symbol:

Problem 20, symbol:

What's in the box?