In Bitcoin terms, simultaneous answers occur frequently, but at the end of the day there can only be one winning answer. When multiple simultaneous answers are presented that are equal to or less than the target number, the Bitcoin network will decide by a simple majority--51%--which miner to honour. Typically, it is the miner who has done the most work, i.e. verifies the most transactions. The losing block then becomes an "orphan block." 
In 1998, Wei Dai published a description of "b-money", characterized as an anonymous, distributed electronic cash system.[12] Shortly thereafter, Nick Szabo described bit gold.[13] Like bitcoin and other cryptocurrencies that would follow it, bit gold (not to be confused with the later gold-based exchange, BitGold) was described as an electronic currency system which required users to complete a proof of work function with solutions being cryptographically put together and published. A currency system based on a reusable proof of work was later created by Hal Finney who followed the work of Dai and Szabo.

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Generally the biggest bitcoin exchanges to buy cryptocurrency will be toward the top of the above list. For example Bitfinex, GDAX, Bitstamp, Coinbase (also the best usd bitcoin exchange) all represent large volume proportions. Daily volume varies, and therefore the world’s largest cryptocurrency exchange vary each day. Go to bitcoinity for a good list of all the best bitcoin exchange site to buy cryptocurrency and their proportional volumes. Some find this handy for arbitrage between markets.

Bitcoin solves the so called ‘’double spending problem’’ present with digital goods. For example, if I have an mp3 file or an ebook on my computer, I can freely copy that file a thousand times and send it to a thousand different people. For a digital currency, the possibility for unlimited copying would mean a quick hyperinflationary death. Bitcoin solves this by maintaining a peer to peer network and recording each transaction in a public ledger called the block chain. Say I send 1 bitcoin from my bitcoin address to my friend John. The bitcoin network records that transaction in the block chain and I no longer have possession of that bitcoin. The coin ‘’moved’’ from my bitcoin wallet to John’s wallet.