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If two different numbers are to be selected from set {1, 2, [#permalink]
29 Jul 2008, 09:37

. If two different numbers are to be selected from set {1, 2, 3, 4, 5, 6}, what is the probability that the sum of two numbers is a perfect square number? A. 1/2 B. 1/3 C. 1/9 D. 2/9 E. 4/9

here what confuses me is , in the total poss cases, do we take ,for eg 1,3 and also along with that 3,1 or should they be considered only once?

. If two different numbers are to be selected from set {1, 2, 3, 4, 5, 6}, what is the probability that the sum of two numbers is a perfect square number? A. 1/2 B. 1/3 C. 1/9 D. 2/9 E. 4/9

here what confuses me is , in the total poss cases, do we take ,for eg 1,3 and also along with that 3,1 or should they be considered only once?

2^2 = 4 and 3^2 = 9 are the only perfect squares we can get as a sum of two numbers from the set. Note that 8 is not a perfect square (8 is a cube, however).

Now, we can either assume order matters, or assume it doesn't- we just need to be consistent. If order matters, there are 6*5 = 30 different pairs of numbers we can choose. Again, if order matters, we can get squares in the following ways: 1,3 3,1 3,6 4,5 5,4 6,3

So the probability should be 6/30 = 1/5. That isn't among the answer choices- where is the question from?
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