Answer: C
1 person in 100 person suffer from disease.
It means 99% person are healthy.
Probability that a person is healthy = 99/100 = 0.99
In a group of 50 people if the test is positive, then we have to perform 50 test individually on each person and 1 group test.
So, probability that in a group of 50 people all are healthy
=> 50C50*(0.99)50 = 0.605
So, the probability that in the group of 50 people at least one people suffer
=> 1- 0.605 = 0.395
Expected number of tests = (51* 0.395) + (0.605*1) = 20.75 = 21 test.