Lesson: Handling Restrictions

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Why do you assume that letters cannot be repeated
gmat-admin's picture

The key word here is "arrange." So, we're taking 5 things (letters) and determining the number of ways to move (arrange) those 5 letters around.

Hi,
I sometimes get a bit confused between mnp rule & !factorial principle. Can you please throw light on the similarities and distinction between them?
gmat-admin's picture

Can you please elaborate. I'm not sure what you mean by the "mnp rule" and the "factorial principle."

Fundamental counting principle vs arranging n unique objects
gmat-admin's picture

We can use either approach, since they both yield the same results.

With the Fundamental Counting Principle, we can see WHY n unique objects can be arranged in n! ways.

Or we can just use the formula (n unique objects can be arranged in n! ways)

why 5*5*4*5*5 is wrong because nothing is mentioned about repetition of words.
gmat-admin's picture

The key word here is "arrange." So, we're taking 5 things (letters) and determining the number of ways to move (arrange) those 5 letters around (with a restriction).

So arrange implies no repetition in that case?

Request you to also explain why have we ruled out repetition in the following question: If all of the telephone extensions in a certain company must be even numbers, and if each of the extensions uses all four of the digits 1, 2, 3, and 6, what is the greatest number of four-digit extensions that the company can have?
gmat-admin's picture

Yes, in that case the word "arrange" implies no repetition.

In your second question, the keyword is "ALL" as in, "...each of the extensions uses ALL four of the digits 1, 2, 3, and 6..."

So, the digits 1, 2, 3 and 6 must ALL be used in the four-digit extension. This means there cannot be any repetition.

Hi Brent,

Can you pls help me understand the solution to the following question, using the above formula:

Seven children — A, B, C, D, E, F, and G — are going to sit in seven chairs in a row. Child A has to sit next to both B & G, with these two children immediately adjacent to here on either side. The other four children can sit in any order in any of the remaining seats. How many possible configurations are there for the children?

(A) 240
(B) 480
(C) 720
(D) 1440
(E) 3600

Thanks, Ipshita
gmat-admin's picture

Happy to help!

I have provided a step-by-step solution here: https://gmatclub.com/forum/seven-children-a-b-c-d-e-f-and-g-are-going-to...

Cheers,
Brent

Hi Brent,

Pls help me understand the solution to this question using the technique explained in the video:

Six children, A, B, C, D, E, and F, are going to sit in six chairs in a row. Child E must be somewhere to the left of child F. How many possible configurations are there for the children?

Thanks, Ipshita
gmat-admin's picture

Hi ipshitasaha,

You'll find my solution here: http://www.beatthegmat.com/six-children-a-b-c-d-e-and-f-t279370.html

Cheers,
Brent

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