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 Post subject: GMAT Probability
PostPosted: Mon Sep 10, 2012 11:42 am 
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Joined: Tue Apr 13, 2010 8:48 am
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There are two decks of 10 cards each. The cards in each deck are labeled from 11 to 20. If we pick a card from each deck at random, what is the probability that the product of the numbers on the picked cards is a multiple of 6?
A. 0.23
B. 0.36
C. 0.40
D. 0.42
E. 0.46

(D) Let’s consider an outcome to be an ordered pair (x, y), where x is selected from the deck #1 and y is selected from the deck #2. There are 10 × 10 = 100 such possible outcomes (pairs).

Let’s count the number of favorable outcomes. First, factorize the given numbers.
11 is prime
12 = 2 × 2 × 3
13 is prime
14 = 2 × 7
15 = 3 × 5
16 = 2 × 2 × 2 × 2
17 is prime
18 = 2 × 3 × 3
19 is prime
20 = 2 × 2 × 5

The product will be a multiple of 6 (6 = 2 × 3) in each of the following cases:
- one of the picked numbers is divisible by 6, which means it has 2 and 3 in its factorization
The numbers in the decks that are divisible by 6 are 12 and 18. There are pairs (12, y), (18, y), (x, 12), (x, 18). Each pair alone gives us 10 favourable outcomes, but outcomes (12, 18), (12, 12), (18,18), (18, 12) are counted twice. So this case gives us 4 × 10 – 4 = 36 favourable outcomes.

- one of the picked numbers is divisible by 2, while the other one is divisible by 3
In this case we do NOT consider numbers that are divisible by 6, because we have already counted those. The numbers that are divisible by 2 are 14, 16, 20. There is only one number which is divisible by 3. It is 15. Therefore this case gives us (3 × 1) × 2 = 6 favourable outcomes.

There are 36 + 6 = 42 favourable outcomes among 100 possible outcomes. Therefore the probability is 42/100 = 0.42. The correct answer is D.
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I think the question should say that the decks are non-identical specifically.


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 Post subject: Re: GMAT Probability
PostPosted: Mon Sep 10, 2012 11:46 am 
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Joined: Fri Apr 09, 2010 2:11 pm
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Quote:
I think the question should say that the decks are non-identical specifically.
No, there is no necessity for this. If you have doubts regarding why we use ordered pairs, then imagine that one of the decks lies to the left and one to the right.


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