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Without repeats there are 4 × 3 = 12 possible 2 digit numbers. With repeats there are 4 × 4 = 16 possible 2 digit numbers.
6 possible 3 digit combonations
Highest 2-digit number = 99 Highest 1-digit number = 9 Highest possible sum from 2, 2-digit numbers = 198 Highest possible sum from 2, 1-digit numbers = 18
There are 10 to the 10th power possibilities of ISBN numbers if d represents a digit from 0 to 9 and repetition of digits are allowed. That means there are 10,000,000,000 ISBN numbers possible.
We're talking about 3-digit numbers that you can make out of 3, 5, 7, and 9.-- The biggest one possible is 975.-- The smallest one possible is 357.-- Their difference is 618 .