1)what is the probability that a seven-digit phone number has one or more repeated digits?

Question

2) five letters, with repetition allowed, are selected from the alphabet. what is the probability that none is repeated?

Answer 1

I will do the second question, they are basically both the same kind.

2) total number of ways with repetition is 26x26x26x26x26
= 26^5

number of ways with no repetition is
26x25x24x23x22

prob none repeated = 26x25x24x23x22/26^5 = .6644

in 1) you will have to subtract the number of non-repeaters from the number of repeaters first.

Answer 2

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