Number of ways to select r items from n, "C^n_r = \\frac{n!}{(r! \\times (n-r)!)}"
Number of bulbs = 24
Number of defective bulbs = 4
Number of working bulbs = 24 - 4 = 20
P(all four bulbs will be defective) = Number of ways to select 4 defective bulbs / Number of ways to select any 4 bulbs
"= \\frac{C^4_4 }{ C^{24}_4} \\\\\n\n= \\frac{1}{10626}"
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