D=event that chip selected is defective d=event that chip selected is NOT defective
Four possible scenarios for the first two selections: P(DDD)=15/100*14/99*13/98=13/4620 P(DdD)=15/100*85/99*14/98=17/924 P(dDD)=85/100*15/99*14/99=17/924 P(ddD)=85/100*84/99*15/98=17/154
Probability of third selection being defective is the sum of all cases, P(XXD)=P(DDD)+P(DdD)+P(dDD)+P(ddD) =3/20