**Disclaimer: not a math expert
**
Not sure if I fully understand the question but I'll give it a shot. I assume we place both bets at the same time? If so we need to take multiple things into account to find the breakeven point, with the chance of team A winning playing a huge role in how often B needs to happen for both bets to break even. As Harrier said:

The bet for team A is break-even if A wins roughly 57.1% of the time.

The bet for thing B is break-even if B happens roughly 47.6% of the time.

However the bet on B happening is dependent on the bet on A winning. After all, if A loses, it doesn't matter how often B happens; we always lose both bets.

Profits of possible outcomes are as follows:

Profit(A loses) = -$20

Profit(A wins, B doesn't happen) = $7.50 - $10 = -$2.50

Profit(A wins, B happens) = $17.50 + $21 = $38.50

That allows us to create the following equation:

EV = [1 - P(A wins)] * -$20 + [P(A wins) * [1 - P(B happens)]] * -$2.50 + [P(A wins) * P(B happens)] * $38.50

EV = -20 + 20x -2.50x + 2.50xy + 38.50xy (x is P(A wins), y is P(B happens)

**EV = 40.5xy + 17.5x - 20**

And then how often B needs to happen to be breakeven depends on the actual probability of team A winning. So say team A wins 100% of the time then the bets are breakeven if:

40.5y + 17.5 - 20 = 0

40.5y = 2.50

y = .0617 = 6.2%

So if A always wins, B needs to happens 6.2% of the time for the bets to be breakeven. You could apply this logic to any 'winrate' for team A.

Interestingly, A needs to win 35% of the time or more to even have a chance of making a profit, although B needs to happen almost 98% of the time in that case for the bets to be breakeven. If A only wins 34% of the time or less, we make a loss even if B happens 100% of the time.

So we can draw the following conclusions:

- If A wins 34% of the time or less, we always make a loss.

- If A wins 35% of the time or more, but B happens less than 6.2% of the time, we always make a loss

- If A wins 35% of the time or more, and B happens more than 98% of the time, we always make a profit

For the chance of B happening between 6.2% and 98%, how often B

**actually** needs to happen for the bets to be breakeven depends very heavily on the actual chance of team A winning and can be found using the above method.

**Long story short: given that we have no information besides payouts (odds) there isn't a single break-even point; there are instead many different ones**
I hope this makes sense (and I hope I'm not wrong lol)