A 'digit' is a numeral - and we have ten of them — 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. After that we re-use them - 10 is not a digit, it is two digits put together.
139
90. all the numbers from 10 through 99
300
There are 6484 digits BETWEEN the two given numbers.
223
25 digits
192 digits
There are 2700 digits.
101
17
There are 900 digits between the numbers 100 and 999. This range includes all the three-digit numbers from 100 to 999, which is calculated by subtracting 100 from 999 and adding 1 (to include both endpoints): 999 - 100 + 1 = 900.
139
19 do.
90. all the numbers from 10 through 99
Twenty of them from 9 to 99
1 googol = 10100 = '1' followed by 100 zeroes. The number has (1 googol + 1) digits.
300