The 0  (single zero) and the 00  (double zero) are the green slots on the roulette wheel. These pockets could receive a 35:1 payout if hit straight up. If there's a split, the payout could be 17:1. This type of bet is somewhat risky, but the payouts could be worth it.

As a general rule, 0 or 00 (the latter being mostly on American wheels but not elsewhere) will pay 35:1 odds just as any other single number bet. Of course, the zero(s) are the key to the house having an edge that guarantees that they make money in the long term.

When you see the odds presented as 30:1 or 3:1 that's actually just showing the payout for a winning bet, not the likelihood of that team winning. 30:1 doesn't mean that the team is 30x more likely to win, it means that if you bet on that team and they win you will receive $30 for every dollar you bet.

On a machine that pays 31-for-1, the machine keeps your bet when you make it, but pays 31 units on a win  still a total of 31 on your side after a win. So 31-for-1 is the same as 30-to-1, 16-for-1 is the same as 15-to-1, 10-for-1 is the same as 9-to-1, and so on.