Five different books (P, Q, R, S, T) are to be arranged on a shelf. The books R and S are to be arranged first second, respectively from the right side of the shelf. The number of different orders in which P, Q and T may be arranged is ________.
6
The problem asks us to determine the number of different orders in which three specific books (P, Q, and T) can be arranged on a shelf, given that two other books (R and S) have fixed positions.
Books Arrangement on a Shelf
We are given five different books: P, Q, R, S, and T. These books need to be arranged on a shelf, which implies a linear arrangement.
There are five positions available on the shelf. Let's represent these positions from left to right:
- Position 1
- Position 2
- Position 3
- Position 4
- Position 5
Shelf Arrangement Details
The problem specifies constraints for books R and S:
- Book R is to be arranged second from the right side of the shelf.
- Book S is to be arranged first from the right side of the shelf.
Let's map these to the shelf positions. Counting from the right:
- The first position from the right is Position 5. So, S is at Position 5.
- The second position from the right is Position 4. So, R is at Position 4.
Therefore, the arrangement on the shelf looks like this, with fixed positions for R and S:
_ _ _ R S
The three empty spaces (Position 1, Position 2, and Position 3 from the left) are now available for the remaining books.
Arranging P, Q, and T Books
After placing R and S in their designated spots, the remaining books are P, Q, and T. These three books must be placed in the three available positions (Position 1, Position 2, and Position 3).
Since we have 3 distinct books (P, Q, T) to arrange in 3 distinct positions, this is a problem of permutations.
Different Orders Calculation
The number of ways to arrange 'n' distinct items in 'n' distinct positions is given by the factorial function, denoted as \(n!\). The formula for \(n!\) is:
\(n! = n \times (n-1) \times (n-2) \times \dots \times 1\)
In this case, we have 3 books (P, Q, T) to arrange, so \(n = 3\).
The number of different orders for P, Q, and T is \(3!\):
\[3! = 3 \times 2 \times 1\]
\[3! = 6\]
These 6 different orders for books P, Q, and T in the first three positions could be:
- PQT
- PTQ
- QPT
- QTP
- TPQ
- TQP
Thus, there are 6 different orders in which P, Q, and T may be arranged given the constraints.
Important Questions from Numerical Computation
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