How to answer it
- Find the group the question limits you to (“given that…”).
- Use that group’s total as the denominator.
- Count the favorable outcomes inside that group.
Worked example
An original question in the SAT format, already answered.
SAT MathWorked example
Of 200 students, 120 play a sport. Of the students who play a sport, 45 also play an instrument.
A student who plays a sport is chosen at random. What is the probability that the student plays an instrument?
- A9/40
- B3/8
- C3/5
- D1/4
Answer: B. The condition limits you to the 120 students who play a sport, and 45 of them play an instrument: 45/120 = 3/8. Choice A uses all 200 students.
The most common trap
Using the whole population as the denominator. The condition shrinks the group.
Practice it
Hourly Prep’s daily sets mix probability and conditional probability with the rest of SAT Math, and send more of it your way when it’s a weak spot. See how the SAT prep plan works, or browse the other SAT skill guides.