To participate in a contest to win cash prizes, a user must pay an entry fee of five (5) Coins, which must be done before the contest begins and the riddle is revealed on the site.
You can then submit up to 4 additional answer entries for that riddle, each at the same entry fee as you paid originally. E.g. Each additional answer entry will cost five (5) coins.
Cash contests have a duration of 30 minutes, and within the contest screen, there is a running timer indicating the amount of time remaining in the contest.