The question is incomplete. The complete question is :
The Jackson family is undecided about whether or not to buy a new car. If the probability is .9 that they will buy one, and if the probability is .3 that they will buy a Ford, and if the probability is .4 that they will purchase a car getting more than 20 miles per gallon, what is the probability that they will buy either a car getting more than 20 miles per gallon or a Ford, if all Fords get more than 20 miles per gallon?
Solution :
Given that :
The probability of buying a new car,
Probability of buying Ford = 0.3
That is, if Jackson family buy a car that is a ford car,
= 0.27
The probability for getting more than 20 miles per gallon = 0.4
That is if Jackson family buy a car that have more than 20 miles per gallon mileage,
The conditions
All of the car have more than 20 miles per gallon mileage.
It means that buying a ford car is subset of getting more than 20 miles per gallon.
Therefore, the probability of buying a car either getting more than 20 miles per gallon or ford =
Therefore,
= 0.36
Thus the probability that Jackson family is buying a car either getting more than 20 miles per gallon or ford is 0.36