1985 AJHSME Problems/Problem 22
Problem
Assume every 7-digit whole number is a possible telephone number except those that begin with or . What fraction of telephone numbers begin with and end with ?
Note: All telephone numbers are 7-digit whole numbers.
Solution
An equivalent problem is finding the probability that a randomly selected telephone number begins with and ends with .
There are possibilities for the first digit in total, and only that works, so the probability the number begins with is
There are possibilities for the last digit, and only that works , so the probability the number ends with is
Since these are independent events, the probability both happens is
See Also
1985 AJHSME (Problems • Answer Key • Resources) | ||
Preceded by Problem 21 |
Followed by Problem 23 | |
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