How would the formula for this be written?

Based on the following tables:

So far, my query looks like this:

Query 4 Create a query that calculates the total amount of time each customer has booked for dog walking. Display the full names of the customers and calculate the amount of time each has booked. Sort the query results to list the best customer first. Save the query as Best Customers. Your output should resemble the format shown in Figure 4-6, but the data will be different