Semi-Bluff question

[foldem]

I was wondering what the probability of your opponent folding needed to be to make raising a better option than just calling a small turn bet in the following semi-bluff situation.

You are the button with A /images/graemlins/heart.gif2 /images/graemlins/heart.gif with a board of Q /images/graemlins/spade.gif 5 /images/graemlins/heart.gif 7 /images/graemlins/heart.gif 2 /images/graemlins/spade.gif

the pot is currently $110 and your lone opponent bets $20 into you (we both have $350 behind). Assume that any A, 2 or heart wins you the pot. What does the probablity of your opponent folding to a raise to $125 need to be in order to make this a higher EV play than just calling his $20 bet? Thanks

[SeattleJake]

If you think you're ahead then it's always a +ev play. If you think he's got a Qh there, then:

130*.30 - 20*.70
130*x + 235*.30*(1-x) - 125*.70*(1-x)

Setting them equal to each other gives x (folding) 21%.