Sample space S = {10, 11, 12, ... 30}
Let E denote the event of choosing a number divisible by 3
Then E = {12, 15, 18, 21, 24, 27, 30} and n(E) = 7
Prob (E) = \(\frac{n(E)}{n(S)}\)
Prob (E) = \(\frac{7}{21}\)
Prob (E) = \(\frac{1}{3}\)