There are cards from 1 to 20 in the box. 3 cards are picked at random. We need to determine the probability the product of the three cards is a multiple of 21.
Now, to get a multiple of 21 we need one card which is a multiple of 3 and another which is a multiple of 7, the third can be any card.
Out of the numbers from 1 to 20, we have 6 numbers that are multiples of 3 and 2 numbers that are a multiple of 7.
If we denote numbers that are divisible by 3 as T, those divisible by 7 as S, and the others as X, the cards can be picked up in any of the following orders: TSX , TXS, XTS, XST, STX and SXT.
The probability of picking up a card with a number divisible by 3 is 6/20, the probability of picking up a card with a number divisible by 7 is 2/20 and the probability of any other number is 18/20.
This gives a probability of (6/20)*(2/20)*(18/20)*6 = .162
Therefore the required probability is 0.162