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 Picking numbers (Posted on 2015-07-17)
Bernardo randomly picks 3 distinct numbers from the set (1; 2; 3; 4; 5; 6; 7; 8; 9) and arranges them in descending order to form a 3-digit number.
Silvia randomly picks 3 distinct numbers from the set (1; 2; 3; 4; 5; 6; 7; 8) and also arranges them in descending order to form a 3-digit number.

What is the probability that Bernardo's number is larger than Silvia's number?
Source: AMO

 See The Solution Submitted by Ady TZIDON No Rating

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 Solution | Comment 5 of 7 |
The numbers are arranged in descending order.

For Bernardo, there are there are 8*7/2 numbers starting with a 9, since he has a choice of 8 numbers for one number, 7 numbers for the other number, and they have to be arranged so as to be in descending order. 8*7/2=28. Every one of these numbers is guaranteed to be greater than any number in Sylvia's set.

The rest of Bernardo's set is exactly the same as Sylvia's. There is a 50/50 chance that Bernardo has the larger number when drawing from this portion of his set.

The total size of Sylvia's set is

(7*6+6*5+5*4+4*3+3*2+2*1)/2 = 56 numbers.

Bernardo has 56+28 = 84 numbers.

He draws from the 9's portion of his set  28/84 of the time with probability 1 that he has the larger number. He draws from the rest of his set 56/84 of the time with a 0.5 probability of having the larger number. The total probability is:

28/84(1)+56/84(0.5) = 56/84 = 2/3 of the time.

 Posted by puzzlesrfun on 2015-07-28 20:27:31

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