Question

A lot of $20$ bulbs contain $4$ defective ones.Suppose the bulb drawn in $\left(i\right)$ is not defective and is not replaced. Now one bulb is drawn at random from the rest. What is the probability that this bulb is not defective ?

Answer 1

Total numbers of bulbs $=20$

One non defective bulb is drawn in

$\left(i\right)$ then the total numbers of bulb left is $19$

Total numbers of events $=19$

Favourable numbers of events $=19-4=15$

Probability that the bulb is not defective $=\frac{15}{19}$