How many whole numbers have exactly three digits?

Answer 1

There are 1000 numbers between 999 and 000

There are 100 numbers between 99 and 00

Subtracting these two values, leaves us with the number of whole numbers with exactly 3 digits.

1000 - 100 = 900
If you are talking about positive integers only (which I assume to be what you meant by "who" numbers), the answer is 900. That would be the numbers 100 through 999 inclusive.

If you are talking about integers, the answer would be 1800. That's 100 through 999 and -100 through -999.

If you want to include all real numbers, it gets a lot bigger. Instead of just having 100 through 999, you'd also have 0.001 through 0.999, as well as 0.01 through 9.99, and 10.1 through 99.9. If we look at only the positives, that's an additional 900 + 900 + 899, or 2699. We also have to double that for negatives, which gives us 5398. That gets added on to our earlier set of 1800, leaving us with a grand total of 7198.

We could make that set even larger, by including imaginary numbers as well (e.g. 123i. The i does not actually qualify as a digit - it's more along the lines of a sign like the "-" symbol). So if you want to include imaginary numbers as well, we have to double our count again, leaving us with a grand total of 14396 numbers.