Final answer:
The approach for calculating the cumulative percentage of cases involves creating group identifiers using findgroups(), applying the sum function to each group using splitapply(), and then calculating the percentage for each day.
Step-by-step explanation:
The question requires applying the findgroups, splitapply, and sum functions on 'washu' dataset for calculating the percentage of total cases in St. Louis Metropolitan Area and remaining counties of Missouri for each day. Below is a general approach one might use: