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How many numbers are there between 500 and 600 in which 9 occurs only once?

 Jul 10, 2021
 #1
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The first digit is 5. 

5 _ _

There are 2 cases.

5 9 a, where a is not 9, or 5 b 9, where b is not 9. 

9 options for a, 9 options for b. 9 + 9 = 18

 

=^._.^=

 Jul 10, 2021

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