Martin's iPod has 2 4 songs on it. He wants to make a playlist consisting of three songs to dance to. Let N be the number of different playlists Martin can make. What are the last 3 digits of N ?
Details and assumptions
The order of the songs matter.
The songs in the playlist should be distinct from each other.
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permutation of 24 things taking 3 at a time. 24P3=22 23 24=12144
N = 24P3 = 12144 , so the last 3 digits of N = 144
There are 24 songs and the playlist could only have 3 songs (with orders matter). We may use permutation in this problem: 24P3=12144 Therefore, the answer to the question is 144.
The number of playlists which can be formed is: 2 4 × 2 3 × 2 2 . Since only the last 3 digits are needed, just multiply 2 4 × 3 × 2 and you'll get 144. ( 4 × 3 × 2 doesn't give you a 3-digit answer)
We can choose 3 songs from 2 4 songs in 2 4 P 3 = 1 2 1 4 4
Now , 1 2 1 4 4 ≡ 1 4 4 ( m o d 1 0 0 )
There is a mistake on the last line. That will be 1 2 1 4 4 ≡ 1 4 4 ( m o d 1 0 0 0 )
Permuation is the key word.
So, P ( 2 4 , 3 )
2 4 × 2 3 × 2 2
They equal 12144. Answer is 1 4 4
12144 ≡ 144 ( mod 1000)
24P3=12144
the last three digits are 144 :)
24P3 = 24 * 23 * 22
According to the given situation, total number of songs are 24.
Martin wants to make a playlist of 3 songs where order of songs matter.
This is similar to the case when we want to select and arrange 3 objects out of 24.
( 2 4 − 3 ) ! 2 4 ! = 2 2 ! 2 4 ! = 1 2 1 4 4
Hence our final answer is 1 4 4
Actually, ( 2 4 − 3 ) ! 2 4 ! = 2 1 ! 2 4 ! , not 2 2 ! 2 4 ! , although you did get the right answer. :/
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We can count systematically, since whatever we choose to be the first song in the playlist cannot be chosen as the second, and whatever we choose for the first and second song cannot be chosen for the third. Thus, by the product rule, the number of possible ordered playlists is 2 4 × ( 2 4 − 1 ) × ( 2 4 − 2 ) = 1 2 1 4 4 , and the last three digits of this is 1 4 4 .