There are 40 beads in total(16+20+4=40). There are 16 black so the probability of getting a black is 16/40=2/5. Since you put the bead back the bag will still have 40 beads when you pick another one. There are 4 white beads so probability of getting a white is 4/40=1/10. Then we multiply these two probabilities 2/5 * 1/10=1/25=4%.
