in Quantitative Aptitude edited by
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1 vote
1 vote

If $4$ Maths books are selected from $6$ different Maths books and $3$ English books are chosen from $5$ different English books, how many ways can the seven books be arranged on a shelf if a Maths is at the beginning and an English book is in the middle of the shelf

  1. $216000$
  2. $218000$
  3. $206000$
  4. $226000$
in Quantitative Aptitude edited by
605 views

2 Answers

2 votes
2 votes

There are seven positions to keep three English books and four Math books, out of which on position $1$ and $4$ we can only place Math and English books, respectively.

Since, available positions are five, we’ve to choose three of them to keep Math books and two of them to keep English books. This can be done in ${5 \choose 3} \times {2 \choose 2}$ ways.

Now, we’ve four positions selected for Math books, we can place Math books in those positions in ${6 \choose 4} \times 4!$ ways.

We also have three positions selected for English books, we can place English books in those positions in ${5 \choose 3} \times 3!$ ways.

Finally, number of ways to arrange books on shelf $= \{ {5 \choose 3} \times {2 \choose 2} \} \times \{ {6 \choose 4} \times 4! \} \times \{ {5 \choose 3} \times 3! \} = \{ 10 \times 1 \} \times \{ 15 \times 24 \} \times \{ 10 \times 6 \} = 216000$

Answer :- A.

1 vote
1 vote

So the answer goes like this:

(select 4 M from 6M) *  (Select 2 M from previous 4M and arrange in 1st and last cell) * (select 2cells from the remaining middle 5 cells and arrange 2 M among them) * (select 3 E from 5 E and arrange them in the remaining middle 3 cells)
=(6C4) * (4C2 * 2C1) * (5C2 * 2C1)  *  (5C3 * 3P3)

=216000 (Ans)

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