Which correctly uses or omits commas? Tony thought the problem seemed simple. After working on it for an hour, however, he changed his mind. Tony thought the problem seemed simple. After working on it for an hour however, he changed his mind.

I think the statement that correctly uses commas is, "Tony thought the problem seemed simple. After working on it for an hour, however, he changed his mind." In this statement, it clearly sends the message that after Tony worked on the problem for an hour, he concluded that is was not an easy problem.