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Out of 40 consecutive integers, two are chosen at random. Find the probability that their sum is odd?

A.

21/40

B.

19/39

C.

21/39

D.

20/39

See Answer & Explanation Lets Discuss
Correct answer is : D
Explanation

Forty consecutive integers will have 20 even and 20 odd integers. The sum of 2 chosen integers will be odd, only if:

  • First is even and second is odd
  • First is odd and second is even
Mathematically the probability will be given by:
P(First is even) * P(Second is odd) + P(First is odd) + P(Second is even)
 
= (20/40) * (20/39) + (20/40) * (20/39)
 
= 20/39
 

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