SOLUTION: How many numbers less than 9000 contain only 6, 7 or 8 as digits?

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Question 1102395: How many numbers less than 9000 contain only 6, 7 or 8 as digits?
Answer by KMST(5328) About Me  (Show Source):
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All positive integer numbers of 1, 2, 3, or 4 digits made with only 6, 7, and 8 are less than 9000.
That includes 3 1-digit numbers: 6, 7, and 8,
3%5E2=9 2-digit numbers: 66, 67, 68, 76,, 77, 78, .... ,
3%5E3=27 3-digit numbers, and
3%5E4=81 4-digit numbers.
The total is
3%2B27%2B9%2B81=30%2B90=120 .