Prepared for 40 Hour Research Internship - Changed to 20
As in 50% of the questions on this forum, the answer to your question is: Talk to your professor. He may have a plan and can advise you on how to proceed. Or he hasn't thought the issue through and talking to you about it will make him articulate what his expectations are and match them with the ones you will have an opportunity to elaborate on this occasion. Or he doesn't have enough money to pay you for 40 hours and that's just the best he can do. 

So many possibilities we all just can't know here but that a short conversation with the person in question can resolve!