There is one railway on the island of Mornia, which runs from Enderby to Widmouth. There are two intermediate stops at Riverford. The train runs continuously from one end of the island to the other at a constant speed, stopping for three minutes at each station. From departing Enderby to arriving at Widmouth takes 42 minutes. From Enderby to Riverford takes 24 minutes. From Maintown to Widmouth takes 36 minutes. How long does it take to get from Maintown to Riverford?