Balvinder wrote:
A group of 8 friends want to play doubles tennis. How many different ways can the group be divided into 4 teams of 2 people?
A. 420
B. 2520
C. 168
D. 90
E. 105
Let the 8 people be: A, B, C, D, E, F, G, and H
Take the task of creating the teams and break it into
stages.
Stage 1: Select a partner for person A
There are 7 people to choose from, so we can complete stage 1 in
7 ways
ASIDE: There are now 6 people remaining. Each time we pair up two people (as we did in stage 1), we'll next focus on the remaining person who comes first ALPHABETICALLY.
For example, if we paired A with B in stage 1, the remaining people would be C, D, E, F, G and H. So, in the next stage, we'd find a partner for person C.
Likewise, if we paired A with E in stage 1, the remaining people would be B, C, D, F, G and H. So, in the next stage, we'd find a partner for person B.
And so on...
Stage 2: Select a partner for the
remaining person who comes first ALPHABETICALLY
There are 5 people remaining, so we can complete this stage in
5 ways.
Stage 3: Select a partner for the remaining person who comes first ALPHABETICALLY
There are 3 people remaining, so we can complete this stage in
3 ways.
Stage 4: Select a partner for the remaining person who comes first ALPHABETICALLY
There is 1 person remaining, so we can complete this stage in
1 way.
By the Fundamental Counting Principle (FCP), we can complete all 4 stages (and thus create 4 pairings) in
(7)(5)(3)(1) ways (= 105 ways)
Answer: E
Note: the FCP can be used to solve the MAJORITY of counting questions on the GMAT. So, be sure to learn it.
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