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52-card lottery (Posted on 2023-03-29) Difficulty: 3 of 5

Let a 52 deck of numbered cards be created as follows:

2 special cards: 0 and 1
25 powers of 2: 2, 4, 8, ..., 2^25
25 powers of 3: 3, 9, 27, ..., 3^25

Shuffle the deck and draw at random 3 cards. Evaluate the product of the 3 numbers, say P.

What is the probability of P=0?
What is the probability of P being a non zero integer square?
What is the probability of P being a 4-digit number?

No Solution Yet Submitted by Ady TZIDON    
Rating: 5.0000 (1 votes)

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Part 2 (analytical corrected) | Comment 4 of 12 |
Part 2)
a) Consider the 1 card to be 2^0.  Then there are 26 powers of 2 in the deck, half of them odd and half of them even.  If we pick three of them, their product will be a power of 2.  Half of those products will be an even power of two and thus a perfect square.  (This occurs when 1 or 3 of the three cards are an even power)  The probability of getting three cards that are powers of two are (26/52)*(25/51)*(24/50) = 6/51.  Half the time (with probability 3/51) this is a perfect square.

b) Similarly, the chance is 3/51 that the product is an even power of 3.
   
c) Or the 3 cards could be two powers of 2 (including 2^0) and an even power of 3 (excluding 3^0).  The probability of picking two powers of 2 and an even power of 3 are (26/52)*(25/51)*(12/50)*3 (because there are only 12 even powers of 3 and it could be picked first, second or third).  This equals 9/51.  But only half of these are a perfect square, because half of product of the powers of two have an even exponent (if both odd or both even).  So the probability of a non-zero perfect square this way is 4.5/51

d) Or the 3 cards could be two powers of 3 (including 3^0) and an even power of 2 (excluding 2^0).  The probability of this happenning and being a perfect square is also 4.5/51
   
   These four are mutually exclusive, and these are the only four ways to get a non-zero perfect square, so the probability of a non-zero perfect square is 3/51 + 3/51 + 4.5/51 + 4.5/51 = 15/51 = 5/17.


Edited on March 30, 2023, 12:54 pm
  Posted by Steve Herman on 2023-03-29 09:00:13

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