The two coins that combine to make 30 cents are a nickel and a quarter. The riddle is very specific in mentioning that only one of the coins is not a nickel, meaning that the second one is a nickel.
This tricky riddle was featured on the American television show "Scrubs." J.D. poses the question to the Janitor. Instead of coming up with the simple answer of a nickel and a quarter, the Janitor researches coin history at the library and determines that the answer is a penny and a rare, out-of-mint coin that costs 29 cents to a specialty collector.