Human blood can be classified into four common blood types-A, B, AB, and O. It is also characterized by the presence (+) or absence (-) of the rhesus factor. The table shows the distribution of blood type and rhesus factor for a group of people. If one of these people who is rhesus negative (-) is chosen at random, the probability that the person has blood type B is 1/9. What is the value of x?

Explanation

The correct answer is 8. In this group,1/9th of the people who are rhesus-negative have blood type B. The total number of people who are rhesus negative in the group is 7 + 2 + 1 + x, and 2 people are rhesus negative with blood type B. Therefore, 2 /(7 + 2 + 1 + x) = 1/9. Combining like terms on the left-hand side of the equation yields 2/(10+x) = 1/9. Multiplying both sides of this equation by 9 yields 18/(10+x) = 1, and multiplying both sides of this equation by (10 + x) yields 18 = 10 + x. Subtracting 10 from both sides of this equation yields 8 = x.