Complex sentence: Viewers sent in postcards with their name, address, and the key they would like to use from 1 to 7-if any of these three elements were missing, the card was rendered void and another was picked.
Split into simpler sentences: Viewers sent in postcards with their name, address, and the key they would like to use, from 1 to 7. If any of these three elements were missing, the card was rendered void and another was picked.