The right answer is about 31.7%. If we had no equity when called, we would need our opponent to fold 40% of the time. Note the huge effect just 4 outs has on how often our opponent has to fold.

Now go into PT3 or HEM and find your turn c-bet success percentage. If you don't have it for PT3, you can download it here C-bet Turn Success Percentage.pt3stat, and then post your findings here in the thread.