24
Most people prefer to write numbers using digits since this is far shorter than writing out the relevant words.
25 digits
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.
99999
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.
There are 3,265,920 numbers that can be formed using all the digits. The answer to the question will depend on which one of these.