899 - 99 = 800 of them
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All the numbers from 10 to 99 are positive 2 digit integers
900 numbers from 100 to 999
There are no pairs of 2-digit numbers with a difference of 90 if both numbers are positive or both negative. However, with one positive and one negative, there are 71 pairs.
There are 320 such numbers.
There are 278 5-digit numbers less than 20,000 that are divisible by both four and nine.