I am going to assume you have all your cases in a table:

Example:

A B C
Case ID Start End
Case1 1/1/05 1/15/05
Case 2 1/1/05
Case 3 1/15/05 1/18/05

You can use a formula called CountA which counts all the cells that have a
value:

Counta(A2:a5)-CountA(c2:c5) would work
That is the total off all cases less the cases solved

Let me know if this works.

Regards,

Stewart Rogers