By applying the mathematics from the Birthday Attack (http://en.wikipedia.org/wiki/Birthday_attack), If an attacker is able to solve 15.8 million of the 180 million captchas, there will be a 50% probability that the attacker can beat the captcha.
I tried refreshing the page 10 times, generating a total of 100 captchas. Out of those, I observed 8 arithmetic problems which I entered into and solved using Wolfram Alpha. That gives roughly the 15.8m/180m necessary to break the captcha with 50% probability.
At 50% probability, again going back to the Birthday Attack mathematics, an attacker would need roughly 16.8 thousand tries before expecting a collision with one they could break.
This probability will increase if an attacker is able to successfully reverse-engineer more patterns.
Edit: thinking about this more after MichaelGG's comment, I think my math is incorrect. Either way, point still stands that Wolfram Alpha can successfully solve 8% of the captchas and other patterns should be solvable by other means too.