In: Statistics and Probability
What is the probability that seven-digit phone number has one or more repeated digits ?
The leading number can not be zero (0).
We have to find the probability that the seven-digit phone number has one or more repeated digits.
We can do this also in this way:
P(one or more repeated digits) ~=~ 1 - P(no digits repeated.)
Let us first find for "P(no digits repeated.)"
Let us get favorable ways of feeding the digits at its place without repetition.
9 ways (from 1 to 9) excluding 0 as leading number | 9 ways (from 0 to 9) excluding the number previously feed, now 0 will be considered. | 8 ways (from 0 to 9) excluding the numbers previously feed. | 7 ways (from 0 to 9) excluding the numbers previously feed. | 6 ways (from 0 to 9) excluding the numbers previously feed. | 5 ways (from 0 to 9) excluding the numbers previously feed. | 4 ways (from 0 to 9) excluding the numbers previously feed. |
So, favorable ways =
Now, total possible ways =
9 ways (from 1 to 9) excluding 0 as the leading number | 10 ways (from 0 to 9) | 10 ways (from 0 to 9) | 10 ways (from 0 to 9) | 10 ways (from 0 to 9) | 10 ways (from 0 to 9) | 10 ways (from 0 to 9) |
thus, total ways =
Thus,
P(one or more repeated digits) ~=~ 1 - P(no digits repeated.)
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