Olympiad Maths Prep

Track / Stage 6 / 320 of 400 #1320 of 2000

Problem 1320

National olympiad, first round
Combinatorics Difficulty 6.7 Find the answer

Jeff has a deck of 1212 cards: 44 LLs, 44 MMs, and 44 TTs. Armaan randomly draws three cards without replacement. The probability that he takes 33 LLs can be written as mn\frac{m}{n}, where mm and nn are relatively prime positive integers. Find m+nm +n.

Official solution

1. First, we need to determine the probability of drawing an LL on the first draw. Since there are 4 LL cards out of a total of 12 cards, the probability is:
P(First L)=412=13 P(\text{First } L) = \frac{4}{12} = \frac{1}{3}

2. Next, we need to determine the probability of drawing an LL on the second draw, given that the first card drawn was an LL. Now, there are 3 LL cards left out of a total of 11 cards:
P(Second LFirst L)=311 P(\text{Second } L \mid \text{First } L) = \frac{3}{11}

3. Finally, we need to determine the probability of drawing an LL on the third draw, given that the first two cards drawn were both LL. Now, there are 2 LL cards left out of a total of 10 cards:
P(Third LFirst and Second L)=210=15 P(\text{Third } L \mid \text{First and Second } L) = \frac{2}{10} = \frac{1}{5}

4. The probability of drawing 3 LL cards in succession is the product of these individual probabilities:
P(Ls)=1331115=1313115=3165=155 P(\text{3 } L \text{s}) = \frac{1}{3} \cdot \frac{3}{11} \cdot \frac{1}{5} = \frac{1 \cdot 3 \cdot 1}{3 \cdot 11 \cdot 5} = \frac{3}{165} = \frac{1}{55}

5. The probability that Armaan draws 3 LL cards can be written as mn\frac{m}{n}, where mm and nn are relatively prime positive integers. Here, m=1m = 1 and n=55n = 55.

6. Therefore, m+n=1+55=56m + n = 1 + 55 = 56.

The final answer is 56\boxed{56}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.