Algebra and Marbles

A couple of days ago I bought 6 bags of marbles.  There are 50 marbles in each bag.  I would like to make at least 3 Algebra activities using these marbles.

Here's one I think is cool:

1. Start with 2 jars, one filled with marbles and the other empty. Give students the weight of the filled jar and also of the empty jar.

2. Then give students a supply of marbles and an empty jar.  I imagine doing this in small groups.

3. Students use algebra to determine approx. weight of 1 marble, 2 marbles etc....

4. The challenge is to find the weight of one marble without weighing anything or opening the first jar.

5. Let them play and problem solve.

6. [y=mx+b] They should discover that the full jar weight = (# of marbles) (weight of one marble) + weight of empty jar. They need to reason it out to get to this point.

7. In order to know how many marbles are in the full jar, they can use the empty jar and fill it with marbles, then count approximately how many marbles are in the first jar. So it's important to use "approximately".

8. Students can solve for weight of one marble using algebra.

I like this activity because they won't feel like they are using algebra until then end.  At that point the algebra should make sense.  What do you think?  Can you come up with any more activities?