Arithmetic on today's supply, not a forecast. It assumes nothing about whether XRP could attract that much money.
A coin's market capitalisation is its price multiplied by the number of coins in circulation. XRP has 62,737,800,504 XRP in circulation at $1.3672 each, which is $85.78B. Holding that supply still and putting Zcash's $19.29B in its place gives $0.30745072 per XRP.
XRP is already worth more than Zcash. Moving to the smaller cap would take XRP down to $0.30745072, which is 0.225x 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 XRP at that level, which is the part that actually decides a price.
$0.30745072 per XRP. That is Zcash's current market capitalisation of $19.29B divided across the 62,737,800,504 XRP in circulation.
0.225x. XRP trades at $1.3672 today and Zcash's market cap is $19.29B against XRP's $85.78B.
No. It is one division: Zcash's market capitalisation spread over the supply of XRP that exists today. It assumes nothing about whether XRP 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 $19.29B of market cap spread over more coins gives a lower price per coin. This uses the 62,737,800,504 XRP in circulation now.