Arithmetic on today's supply, not a forecast. It assumes nothing about whether Ethereum could attract that much money.
A coin's market capitalisation is its price multiplied by the number of coins in circulation. Ethereum has 122,015,220 ETH in circulation at $2,520.53 each, which is $307.54B. Holding that supply still and putting XRP's $85.78B in its place gives $702.9871 per ETH.
Ethereum is already worth more than XRP. Moving to the smaller cap would take ETH down to $702.9871, which is 0.279x of what it trades at now.
Supply is held still, and in practice it moves: coins are minted, burned and unlocked, and every one of those changes the price a given market capitalisation implies. The figure also says nothing about whether the money exists to buy ETH at that level, which is the part that actually decides a price.
$702.9871 per ETH. That is XRP's current market capitalisation of $85.78B divided across the 122,015,220 ETH in circulation.
0.279x. Ethereum trades at $2,520.53 today and XRP's market cap is $85.78B against Ethereum's $307.54B.
No. It is one division: XRP's market capitalisation spread over the supply of ETH that exists today. It assumes nothing about whether Ethereum could attract that much money, and it holds the supply still when in practice supply changes.
It sets it. Market cap is price times circulating supply, so the same $85.78B of market cap spread over more coins gives a lower price per coin. This uses the 122,015,220 ETH in circulation now.