...so what is the answer to 27 : 24 :: 64 : ?
64 is 4 * 4 * 4 and _60_ is (4 - 1) * 4 * (4 + 1)
Given that they've defined "::" differently to the usual "as" the answer could be anything. More specifically, the answer is f(64), where f(x) is wharever was in the head of the guy who made the question.
The same goes for the "what is the next number in this sequence" questions: unknowable given the information provided.
But personally, if asked this question suddenly, I would've gone for the simplest: 61, because 27 - 24 = 64 - 61.