24
Most people prefer to write numbers using digits since this is far shorter than writing out the relevant words.
25 digits
99999
You can select 9 numbers for the first digit, 8 numbers for the second digit, and 7numbers for the third digit; so 504 (e.g. 9*8*7) different three digit numbers can be written using the digits 1 through 9.
60
Six, if you don't repeat any.
depends on your answer
There are 60480 numbers.
Using the digits of 1345678, there are 210 three digit numbers in which no digit is repeated.
In decimal we write a number by using a combination of 10 digits (0-9). In base 2, however, numbers are written by using a combination of only 2 digits (0 & 1). We call this number system binary.
There are 4^6 = 4096 such numbers.
If the digits are all different then 18. Otherwise, 192.