There are a few different types of auctions that could be used in order to auction off a 20 dollar bill. The most common type of auction is the English auction, which is the standard auction used in most situations. In an English auction, bidding starts at a low price and gradually increases until only one bidder remains. This type of auction would likely be the best option for a 20 dollar bill, as it would start at a low price and then gradually increase until someone is willing to pay the full amount.
Another type of auction is the Dutch auction, which is less common than the English auction. In a Dutch auction, the item is put up for sale at a high price and then the price is lowered until someone is willing to buy it. This type of auction could also be used for a 20 dollar bill, but it is less likely to get the full amount than an English auction would.