If I roll a fair, regular six-sided die four times, what is the probability that I will roll the number 3 exactly once?

[4 nCr 1] * 5^3 =[4 x 125] =500 / 6^4 =125 / 324 = 38.58%

OR:

[4 nCr 1] * (1/6)^1 * (5/6)^3 =38.58%