There are 20 10% off cards, 10 15% off cards, 6 20% off cards and 4 50% off cards. What is the probability that a person will draw a 50% off card on the third draw?

To find the probability of drawing a 50% off card on the third draw, we need to know the total number of cards available for selection on each draw. Let's calculate it step by step:

First, let's find the total number of cards available on the first draw. Adding up the quantities of each type of card, we have 20 + 10 + 6 + 4 = 40 cards in total.

For the second draw, we need to take into account that on the first draw, one card has already been selected. So, the total number of cards available on the second draw is 40 - 1 = 39.

Finally, on the third draw, we need to consider that two cards have already been selected on the previous draws. Therefore, the total number of cards available on the third draw is 40 - 1 - 1 = 38.

Now, let's calculate the probability of drawing a 50% off card on the third draw. Out of the 4 total 50% off cards, there is only 1 left on the third draw.

The probability of drawing a 50% off card on the third draw can be calculated as:

Probability = (Number of favorable outcomes) / (Total number of possible outcomes)

Probability = 1 (number of 50% off cards) / 38 (total number of cards available on the third draw)

Probability = 1/38 ≈ 0.0263 or 2.63%

So, there is approximately a 2.63% chance that a person will draw a 50% off card on the third draw.