# permutation problem 1

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• Apr 26th 2009, 07:01 AM
wheresthecake
permutation problem 1
hey guys, need some help with this problem. this is one of four permutation problems that our teacher gave us to solve but haven't really figured it out, plus i was sick so i missed the lecture..

1) Suppose a student lives in a dormitory.

A: In the common area there are 20 books and 24 magazines. How many ways can we choose either? and having chosen one of each, how many ways can we choose another book and magazine?

B: If there are 8 outside doors in the dorm, how many ways can a student enter one and leave by b1) a different door and b2) any door?

thanks for the help. we've been given all the basic formula for permutations but I'm at a loss at how to apply them still.
• Apr 26th 2009, 09:57 PM
SengNee
Quote:

Originally Posted by wheresthecake
hey guys, need some help with this problem. this is one of four permutation problems that our teacher gave us to solve but haven't really figured it out, plus i was sick so i missed the lecture..

1) Suppose a student lives in a dormitory.

A: In the common area there are 20 books and 24 magazines. How many ways can we choose either? and having chosen one of each, how many ways can we choose another book and magazine?

B: If there are 8 outside doors in the dorm, how many ways can a student enter one and leave by b1) a different door and b2) any door?

thanks for the help. we've been given all the basic formula for permutations but I'm at a loss at how to apply them still.

A: My english is poor, I can't see the question clearly.(Worried)

B:
1)
Enter: 8 doors
Exit: 7 doors(cannot use the previous door)
$^8P_1 \cdot ^7P_1$1
or
$^8C_1 \cdot ^7C_1$
or
8 times 7

2)
Enter: 8 doors
Exit: 8 doors(can use the previous door)
$^8P_1 \cdot ^8P_1$1
or
$^8C_1 \cdot ^8C_1$
or
8 times 8
or
$8^2$