Solution : First ten letters of the alphabet can be used i.e A,B, C,D,E, F,G,H, I,J. Four letter code is to be formed.
The first place [from left] can be filled by using any of the 10 alphabets. So, there are 10 ways to fill first place.
Since, repetition is not allowed . So, second place can be filled by remaining 9 alphabets.
So, there are 9 ways to fill second place.
Similarly, third place can be filled by remaining 8 alphabets.
And fourth place can be filled by remaining 7 alphabets.
Hence, the required number of ways in which four letter code can be formed is
=10×9×8×7
=5040
Mark M. answered • 09/30/18
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If a letter can be used more than once, then the number of possibilities is [4][4][4][4] = 44 = 256
If each letter is used only once, then the number of possibilities is [4][3][2][1] = 24
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Arthur D. answered • 09/30/18
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if there is repetition...
there are 4 letters, all different
4*4*4*4=256
if there is no repetition...
4*3*2*1=24
this is a permutation, not a combination
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