Brent@GMATPrepNow wrote: ↑Mon May 11, 2020 8:31 am
Set A {1, 2, 3, 4, 5}
Set B {1, 2, 3, 4, 5, 6, 7}
Bill randomly selects a number from set A, and Sue randomly selects a number from set B. What is the probability that Sue’s number is greater than Bill’s number?
A) 4/7
B) 7/12
C) 3/5
D) 2/3
E) 5/7
Answer: A
Source:
www.gmatprepnow.com
When we solve this probability question using counting techniques, we quickly see it's a lot of work to (accurately)
list and count all the possible outcomes in which Sue's number is greater than Bill's number.
Here's a technique that you may be able to apply with other questions requiring listing in accounting.
GIVEN:
Set A {1, 2, 3, 4, 5} and
Set B {1, 2, 3, 4, 5, 6, 7}
We're selecting one number from the 5 numbers in set A
And we're selecting one number from the 7 numbers in set B
So, the total number of possible outcomes = (5)(7) =
35
To determine the number of outcomes in which Sue's number is greater than Bill's number, we can create a table that looks like this:
Note: each of the 35 boxes represents one possible outcome.
For example, the box with the star (below) represents the outcome in which Bill select the
1 from set A, and Sue selects the
2 from set B
So, if we place a star in every box in which
Sue's number is greater than
Bill's number, we get:
So the total number of outcomes in which Sue's number is greater than Bill's number =
1 + 2 + 3 + 4 + 5 + 5 =
20
So, P(Sue's number is greater than Bill's number) =
20/
35 = 4/7
Answer: A
Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
