Given :
4 out of 20 bulbs are defective (i.e.)
Number of defective bulbs = 4
Number of non-defective bulbs = 20 – 4 = 16
If a bulb is drawn at random, the total outcomes are = 20
Number of outcomes favourable to ‘defective bulb’ = 4
∴ Probability of getting a defective bulb
= \(\frac{No.\,of\,favourable\,outcomes}{Total\,no.\,of\,outcomes}\)
\(=\frac{4}{20}=\frac{1}{5}\)
Suppose a non-defective bulb is drawn and not replaced, then the bulbs remaining are = 19
∴ Total outcomes in drawing a bulb from the rest = 19
Number of favourable outcomes in drawing nondefective bulb from the rest = 16 – 1 = 15
∴ Probability of getting a non-defective bulb in the second draw
= \(\frac{No.\,of\,favourable\,outcomes}{Total\,no.\,of\,outcomes}\)
= 15/19