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In how many ways can 3 distinct numbers be chosen from the set { 1 , 2 , 3 , , 10 } \{1, 2, 3, \ldots, 10 \} such that the sum of the numbers is odd?


The answer is 60.

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1 solution

Pranshu Gaba
Apr 29, 2016

If the sum of three integers is odd, then two cases are possible:

  1. All three numbers are odd.
  2. Two numbers are even, and the third number is odd.

To pick three distinct odd numbers from the given set, there are ( 5 3 ) = 5 ! 2 ! × 3 ! = 10 \dbinom{5}{3} = \dfrac{5!}{2! \times 3!} = 10 ways.

To pick two even and one odd number from the set, there are ( 5 2 ) × ( 5 1 ) = 50 \dbinom{5}{2} \times \dbinom{5}{1} = 50 ways.

Therefore, the three numbers can be chosen in 10 + 50 = 60 10 + 50 = \boxed{60} ways. _\square

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