not sure why you have a greatest integer, since this is an integer valued rv
Suppose three fair dice are tossed repeatedly. Let the random variable X denote the roll on which a sum of 4 appears for the first time. Use the expression Fx(t) = P(X <= t) = 1 - (1 - p)^[t], where [t] denoted the greatest integer in t, t>=0, to evaluate P(65 <= X <= 75).