Arithmatical - Probability

CATEGORY

DIRECTIONS : Problems based on Probability.
16. A box contains 20 electric bulbs, out of which 4 are defective. Two bulbs are chosen at random from this box. The probability that at least one of these is defective is
  A.  4/19
  B.  7/19
  C.  12/19
  D.  21/95
Solution
P( None is defective)
= 16C2 / 20C2
= (16x15/2x1 ×2x1/20x19)
= 12/19.
P( at least one is defective)
= (1- 12/19)
= 7/19.
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