The employee spilled coffee on the table, making the entry for hour 2 unreadable. the employee knows that the mean for the data in the table is 21. how many prescriptions were filled during hour 2?

We are given a set of data for each hour as well as the average of the data. One of the data is made unreadable. To get this data, we simply have to use the formula for the average which is
average = (data1 + data2 + data3 + ... + datan) / n
where n is the total number of data
Since only data2 is made unreadable, the rest of the data are available and the value for data2 can be easily determined.