Final answer:
To correctly combine the sentences, a semicolon should be used between "On Saturday, I volunteered as a tutor at my local community center" and "as usual, the youngest children were the most energetic," resulting in the sentence being "On Saturday, I volunteered as a tutor at my local community center; as usual, the youngest children were the most energetic."
Step-by-step explanation:
The correct way to combine the sentences "On Saturday, I volunteered as a tutor at my local community center" and "As usual, the youngest children were the most energetic" is to use a semicolon to link two independent clauses that are closely related in content. This rule applies when you do not use a conjunction (such as 'and' or 'but'). Therefore, the correct sentence is: "On Saturday, I volunteered as a tutor at my local community center; as usual, the youngest children were the most energetic." This keeps the clarity of both original sentences while combining them into one cohesive thought.