First slide

Theorems of probability

Question

Three electric lamps are fitted in a room. 3 bulbs are chosen at random from 20 bulbs having 16 good bulbs. The probability that the room is lighted is

Easy

A

$\frac{282}{285}$
B

$\frac{283}{285}$
C

$\frac{284}{285}$
D

$\frac{281}{285}$

Solution

No. of bad lights (bulbs)=4
$P (r o o m i n l i g h t e d) = 1 - P (n o . l i g h t i n g)$

= $1 - \frac{4_{c_{_{_{3}}}}}{20_{c_{_{3}}}}$

= $\frac{284}{285}$

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