You have the numbers 1–24 written on slips of paper. If you choose one slip at random, what is the probability that you will not select a number which is divisible by 3?

Question

unklerhaukus · Answer

$$ {\{3,6,9,12,15,18,21,24\} \over \{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24 \}} $$
$$=1/3$$

unklerhaukus · Answer

that is 1/3 probability you select a number divisible by three,
the probability that the number is not divisible by three is the total probability - 1/3
$$=1-1/3=2/3$$