SOLUTION: How many positive 3-digit integers have a units (ones) digit of 9?

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Question 1189187: How many positive 3-digit integers have a units (ones) digit of 9?
Found 2 solutions by Alan3354, ikleyn:
Answer by Alan3354(69443) About Me  (Show Source):
Answer by ikleyn(52756) About Me  (Show Source):
You can put this solution on YOUR website!
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First (left-most) digit can be any from 1 to 9.


Next (tens) digit can be any from 0 to 9.


Therefore, there are 9*10 = 90 such three-digit numbers.

Solved.

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Even more simple reasoning is possible. 

Consider the set of all such 3-digit numbers. 
Cut the last digit: you will get the set of all 2-digit numbers.

The cardinality of this set of 2-digit numbers is 90 - - - it gives the answer.