Question 339817: Consider the old rules for telephone are codes determined by creating three digit codes with 0 or 1 as the second digit but excluding codes ending in 00 or 11 as these were reserved for toll-free calls, emergency services, and other special uses. How many different codes were possible under these rules?
Answer by jrfrunner(365)   (Show Source):
You can put this solution on YOUR website! based on how you defined the problem, there are 3 numbers to choose
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first number can be any digit so there are 10 choices
second number can be only 0 or 1 so there are 2 choices
third number can not include 0 if the second number is 0 or cannot include 1 if the second number is a 1, so there are 9 choices available
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answer: 10*2*9=180
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