Answer
See below.
Work Step by Step
a) $P(induced \ substance)=P(induced \ substance|outside \ factors)P(outside \ factors)=0.13\cdot0.73=0.0949$
b) $P(disease \ or \ infection)=P(disease \ or \ infection|natural\ occurence)P(natural\ occurence)=(0.27+0.17)\cdot0.87=0.3828$