This is a 2 part Rollup. The first part captures the number activities on an opportunity. The second part makes use of workflow rules to capture the number of activities completed before a deal was closed. Take note if an opportunity is re-opened and closed the number of activities before closed...

Business Problem:  

Salesforce does not have a way of notifying anyone when the number of cases in the current month reaches a predefined limit for an account. 

A great way to use Rollup Helper is to combine the target fields that Rollup Helper calculates with workflow rules. For example, when projects go over budget or are delayed, you will want to have access to this information at the project level to use in a workflow rule.