Subjective probabilities

1)British scientists have developed a 15 minute brain scan they hope could be used to detect autism in children.Autism and related disorders affect up to 7 out of every 1000 individuals. The brain scan method was 90% accurate in correctly identifying the autistic patients.It also showed a negative result for healthy controls in 80% of cases.If a child tested positive for autism,what is the probability that he indeed has autism?

Pick the option CLOSEST TO THE ANSWER:
(a) 0.9 (b) 0.7 (c) 0.5 (d) 0.3 (e) 0.1



2)Determine which statements (A,B,C) has the highest probability of being true.

A:A car traveling at 120km per hour on a 2-lane expressway will have an accident
B:A car traveling at 120km per hour on a 2-lane expressway will have a major accident
C:A car traveling at 120km per hour on a 2-lane expressway will have a major accident in which the driver will be killed instantly

OPTIONS:
1) Subjective probabilities vary from individual to individual.So either one of the three statements can be most probable
2)We need statistics about car accident expressway before we can asses which statement is most probable 
3)Statement A is most probable
4)Statement B is most probable
5)Statement C is most probable