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A box contains nine bulbs out of which 4 are defective. If four bulbs are chosen at random, find the probability that all the four bulbs are defective.

A box contains nine bulbs out of which 4 are defective. If four bulbs are chosen at random, find the probability that all the four bulbs are defective. Correct Answer $$\frac{{1}}{{126}}$$

Out of nine, five are good and four are defective.
Required probability
$$\eqalign{ & = \frac{{{}^4{C_4}}}{{{}^9{C_4}}} \cr & = \frac{1}{{126}} \cr} $$

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