Calculation Triangle

This virtual manipulative encourage the development of flexible arithmetic thinking.
Watch a video of the app: http://www.youtube.com/watch?v=iN3b20F6UC8
A calculation triangle is splited into three fields. Each field can contain tokens. On each edge the sum of the two corresponding fields is written.
And this is how it works:
Touch with you fingers on a field to produce tokens and move them from one field to another.
Move tokens out of the triangle to delete them.
Close the shutter to hide a field.
If you swipe over a number you can hide it or show it again.
Shake the calculation triangle to emty it.

This virtual manipulative does not include tasks. But you can use this app to create puzzle problems for a learning partner or to model and solve existing tasks. Here are some exemples for good tasks to effectively work with this app:
Find the missing numbers?
Figure out how many tokens are on the hidden field?
How do the numbers change, if you ...
... add/remove one (two, three, ...) token to each field?
... move one (two, three, ...) token from one field to another?
... add one token to two of the three fields?
...
Is it possible to have three odd numbers? Why (not)?
Is there a relationship between the sum of the numbers and the total quanitiy of tokens on the fields?
Is it possible to produce any numbers or are there restrictions?