Question blood-
| Blood type | ||||
| Rhesus factor | A | B | AB | O |
 | 33 | 9 | 3 | 37 |
 | 7 | 2 | 1 | x |
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 above 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 
. What is the value of x ?
Blood type Rhesus factor A B AB O 33 9 3 37 7 2 1 x
Hard-difficulty · SAT Math · Probability & Conditional Probability — Conditional probability formula. Read the question above, select your answer, and check the full explanation below to understand exactly why the correct choice works.
Answer explanation
The correct answer is 8. In this group, of the people who are rhesus negative have blood type B. The total number of people who are rhesus negative in the group is 
, and there are 2 people who are rhesus negative with blood type B. Therefore, 
. Combining like terms on the left-hand side of the equation yields 
. Multiplying both sides of this equation by 9 yields 
, and multiplying both sides of this equation by 
yields 
. Subtracting 10 from both sides of this equation yields 
.
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