Cards are drawn one-by-one at random from a well-shuffled pack of 52 playing cards until 2 aces are obtained
from the first time. The probability that 18 draws are required for this is
18 draws are required for 2 aces. This means in the first 17 draws, there is one ace and 16 other cards, and 18th
draw produces an ace.
so, the required probability is