Multiple choice

A bag contains 12 Green and 8 White balls. Four balls are drawn out one by one and not replaced. What is the probability that they are in alternate colours?

  1. 308/4845

  2. 317/4845

  3. 414/4845

  4. 407/4845

  5. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Two alternating color patterns possible: G-W-G-W or W-G-W-G. For G-W-G-W: (12/20)(8/19)(11/18)(7/17). For W-G-W-G: (8/20)(12/19)(7/18)(11/17). Notice numerators are same (12*8*11*7). Total = 2*(12*8*11*7)/(20*19*18*17) = 2*7392/116280 = 14784/116280 = 317/4845. B matches this.

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