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02 Nov 2021, 04:30
Kudos
Bookmarks
D
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Difficulty:
5%
(low)
Question Stats:
89% (01:01) correct
11%
(02:20)
wrong
based on 120
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Not Attempted Yet
Out of seven different pens that I have, each day I take two pens to the office. If the combinations of the two pens on any given day is not the same as that on any other day, find the numbers of days that are required to exhaust all such combinations.
(A) 7
(B) 14
(C) 20
(D) 21
(E) 22
(A) 7
(B) 14
(C) 20
(D) 21
(E) 22
07 Nov 2021, 08:13
Kudos
Bookmarks
Bunuel
In other words, we want to determine the total number of ways to select 2 pens out of 7 pens.
Since the order in which we select the pens does not matter, we can use combinations.
We can select 2 pens out of 7 pens in 7C2 ways (= 21 ways)
So, we can bring a different pair of pens each day for 21 days.
Answer: D
ASIDE: Here's a video lesson on calculating combinations (e.g., 7C2) in your head:
