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 63,389,767,516 XRP in circulation at $1.2913 each, which is $81.86B. Holding that supply still and putting Zcash's $21.05B in its place gives $0.33208733 per XRP.
XRP is already worth more than Zcash. Moving to the smaller cap would take XRP down to $0.33208733, which is 0.257x 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.
$0.33208733 per XRP. That is Zcash's current market capitalisation of $21.05B divided across the 63,389,767,516 XRP in circulation.
0.257x. XRP trades at $1.2913 today and Zcash's market cap is $21.05B against XRP's $81.86B.
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 $21.05B of market cap spread over more coins gives a lower price per coin. This uses the 63,389,767,516 XRP in circulation now.