Manhattan Prep Official ExplanationA company is forming a three-person leadership committee from a pool of eight qualified candidates: Aly, Ben, Cam, Dan, Emi, Fay, Gia, and Hal. Due to conflicts of interest, two pairs of candidates are incompatible: Aly and Ben cannot both serve on the committee together, and Cam and Dan cannot both serve on the committee together. A committee is considered valid only if its three members do not include either of these pairs of incompatible candidates.
Consider the following incomplete sentence:
If three candidates are chosen at random, the probability that the selected candidates form a valid committee is X / Y , where the fraction is expressed in simplest form.
Select for X and Y values that are consistent with the information provided. Make only two selections, one in each column.
Step 1: Understand the Prompt and QuestionThe prompt describes a committee selection process with two incompatibility constraints and asks for the probability that a randomly chosen committee is valid. Because we need to count outcomes in order to find a probability, this is a
Probability and
Combinatorics problem.
The key formula is:
P(event) = Favorable Outcomes / Total Outcomes
To find this probability, we need to count the total number of possible committees and the number of valid ones — those that include neither forbidden pair. Rather than counting valid committees directly, it may be easier to count the invalid ones and subtract from the total.
Step 2: Plan your ApproachUse the
Anagram Method to count the total number of possible committees: assign each candidate a letter —
Y (selected) or
N (not selected) — and count the distinct combinations using the formula \(\frac{?????!}{?!?!}\), where
Y and
N are the number of candidates assigned each of those letters, respectively. To calculate the number of valid committees, count the number of committees that contain a forbidden pair and subtract from the total number of possible committees.
Step 3: Solve the ProblemTo find the total number of committees possible that select 3 people from 8 candidates, count arrangements of a word with 3
Y’s and 5
N’s:
Total = 8!/(5!3!) = 56
Next, find the number of invalid committees by counting how many committees contain a forbidden pair. If a committee contains Aly and Ben, there is only one more spot to be filled and six possible candidates to fill that spot, so there are a total of six committees that are invalid because of the Aly/Ben pair. The same logic applies for Cam and Dan: there are another six committees that are invalid because of the Cam/Dan pair. Out of 56 possible committees, 6 + 6 = 12 are invalid, leaving 56 - 12 = 44 valid committees. To compute the probability a randomly-selected committee is valid:
P(valid) = # Valid Committees / # Total Possible Committees
P(valid) = 44 / 56 = 11 / 14
The correct answer is
11 for the first column and
14 for the second column.
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