A lot of 20 bulbs contain 4 defective ones. One bulb is drawn at random from the lot. What is the probability that this bulb is defective?

i) P(Defective bulb) = Defective Bulbs / Total Bulbs = 4/20 = 1/5
ii) Since one bulb is already drawn, the number of bulbs remaining is 19. Number of non-defective bulbs is 16 (as one defective bulb is picked). P(Non-defective) = 16/19.