Answer: 87.5%
Step-by-step explanation:
Let say that there're 100 people in the town.
10% of population were dissatisfied with plumbing jobs => that's 10 people.
Half of them deal with Plumber A => 5 people dissatisfied with plumber A.
A does 40% of the plumbing jobs in the town => 40 people use A's service.
=> The probability that a comsumer will obtain a satisfactory plumbing job, given that the plumber was A is: (40-5)/40 = 87.5%