Final answer:
The ending balance in the Service Revenue account would be $20,000.
Step-by-step explanation:
The ending balance in the Service Revenue account can be calculated by adding up the revenue generated from the transactions specified. In this case, the revenue recorded from billing a client for design services completed is $12,000. Additionally, the revenue received on account for previously recorded services is $8,000. Therefore, the ending balance in the Service Revenue account would be $12,000 (from billing a client) + $8,000 (from receiving payment on account) = $20,000.