Hypothetical scenario:

You can bet on a single dye roll (choosing a number between 1-6), but if you win you get paid x12 of your stake (instead of x6).
So the Expected Value is positive (see example below), but you'd still have 83% chance of losing.

1 - will you take that bet?
2 - if so, what % of your available funds would you put at stake (i.e. funds you're willing to gamble and afford to lose)?

Again, this is a single, non-repetitive bet.

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Simulation for BTC1 bet:

EV = (83% x -BTC1) + (17% x BTC12) = +BTC1.21