I think this is the best approach. Especially when there are multiple situations that satisfy the question. In this question, it was simple, there are only 4 possble results RR, RG, GR, GG, and the only one that doesn't satisfy the solution is RR. So rather than finding the probability of RG, GR, and GG that DOES satisfy the "at least 1 green hat". If we find the situations that DO satisfy, meaning all 3 situations, then we have to add together their total probability. If we find the probability of the 1 situation we DO NOT want, all we have to do is subtract that probability from 1 and it's a much simpler problem. I would say much simpler than a 700 level question.
For me easiest way is 1-(RR)=1-6/11*5/10=8/11
yeap! have the same...