~는 거라고 풀 수가 있지 is grammatically okay, but it would be very rare to hear such a phrase. 풀다 means "untangle/solve (a knot, math problem, thorny issue, etc)", so it's not an idiomatic phrase. What you heard is probably ~는 거라고 볼 수가 있지, which is a common idiom. 보다(to see) is very often used in the sense of "to regard/consider/view", just as "see" is in English. So (~는 거라고 / ~ㄴ다고 / ~(이)라고) 볼 수(가) 있다 means "(it) can be considered/thought to be ...". 보다 in these phrases can be replaced with 생각하다(think) or 말하다(say, state). Examples: - 이 문제는 해결되었 다고 볼 수 있습니다. This problem can be considered solved. - 꼭 그렇게만 볼 수는 없어요. That shouldn't be the only way to view/approach it. - 저는 이 상황을 이렇게 봅니다. I view/take the situation this way. ~고 한다 is fairly close in meaning to ~볼 수 있다, but not quite the same. ~고 한다 is casually quoting a speech or a general fact, like "it is said that ..." or "They say that ...". ~고 볼 수 있다 on the other hand states a viewpoint or an approach to an issue, with an overtone of critical thinking.

안녕하세요! :) Hi~! Actually "~는 거라고 풀수가 있지" sounds little bit weird to me and this is because, considering the meaning of "풀 수가", it sounds like "you can solve that ~" You indicated that it was translated as "you can say that" so instead of former one, "~라고/이라 말할 수가 있지" would be better phrase. Lastly, as for interchange, you can use both of them. But honestly, former one is more likely to contain your confidence. And the last one is sounds like you are just deliverying some pharses to someone. I'm little bit worry about my reply's grammer so that you may have some trouble to understand. But no matter what ,I really hope that this could be helpful to you! :)