I am really stuck on this brain teaser I was wondering if someone could help me. Thanks in advance?

A major fast-food chain held a contest to promote sales. With each purchase, a customer was given a card with a whole number less than 100 on it. A $100 prize was given to any person who presented cards whose numbers totaled 100. Jamie collected cards containing 3,9,12,15,18,27,51,72,84. Can you find a winning combination? Can you suggest how the contest could be structured so there would be at most 1000 winners throughout the country?