2005 AMC 10B Problem 18
Problem 18 of 25IntermediateCombinatorics
All of David’s telephone numbers have the form where and are distinct digits and in increasing order, and none is either or How many different telephone numbers can David have?
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Solution
The seven digits are chosen from and once chosen they must be written in increasing order, so only the choice of digits matters.
Choosing seven of these eight digits is the same as choosing the one digit to leave out, which can be done in ways.
Thus, D is the correct answer.
Tagged: combinations · bijection