What would be a formula for the following in MS Access?

The table for which this question is based on contains the following information:

And the query I have so far looks like this:

Create a query that calculates the amount of money earned by each walker. Display the full names of the walkers and calculate the amount of money each has earned from the business. Keep in mind that you charge $20 for each half-hour of walking. Save the query as Money Earned by Walker. Your output should resemble the format shown in Figure 4-5, but the data will be different. Create a query that calculates the amount of money earned by each walker. Display the full names of the walkers and calculate the amount of money each has earned from the business. Keep in mind that you charge $20 for each half-hour of walking. Save the query as Money Earned by Walker. Your output should resemble the format shown in Figure 4-5, but the data will be different