Final answer:
In a single treatment session where 20 minutes are spent on therapeutic exercise and 20 minutes are spent on ultrasound, a total of 2 units can be billed.
Step-by-step explanation:
The total units that can be billed for a single treatment session where you spend 20 minutes providing therapeutic exercise and 20 minutes providing ultrasound is calculated based on time. In healthcare billing, one unit is typically equal to 15 minutes. Therefore, the 20 minutes of therapeutic exercise would be counted as 1 unit, and the 20 minutes of ultrasound would also be counted as 1 unit. So, the total units that can be billed for this session would be 2 units.