depends on what you want
the least amount of robux possible that you can buy
400 what?
You can donate whatever amount of Robux/Tickets to BC members but you can donate 400 Robux by buying BC yourself and referring the user you want to donate.
400 days 400 nights
You don't get free Robux unless you are part of a group that you actively contribute in. As far as Tix, they have been discontinued long ago.
Depending on how much money the Roblox card you bought is worth the number of robux you can get from it varies. For $5 you can get about 400 robux. For $20 You can get about 1900 robux.
400 years
400 hours
Depending on the price of the card the card can be with various amounts. You can purchase 5, 10, and 20 dollar Roblox cards and they are worth various amounts of Robux. One robux is worth a little more than a penny, so $5 is worth about 400 robux, $10 is worth a 1000 robux, and so on.
400 divide by 60 walla
It would take about 400 hits on the barrel