6
You can select 12 numbers for the first digit, 11 numbers for the second digit, and 10 numbers for the third digit; so 12*11*10 = 1320 sets of 3 numbers can be made out of 12 different numbers.
9
210
He wore number 49.
If the numbers are not repeated then the highest 5 digit even number to be made out of 0123456789 is : 98764.
36 of them.
You have 5 choices for the 1st digit of the number. 4 choices for the 2nd digit and 3 choices for the 3rd and final digit. This means that there are 5 x 4 x 3 = 60 3-digit numbers that can be made from 5 different digits subject to each digit only being used once.
If there are no restrictions on duplicated numbers or other patterns of numbers then there are 10 ways of selecting the first digit and also 10 ways of selecting the second digit. The number of combinations is therefore 10 x 10 = 100.
The first digit can by selectd in one of two ways, since a number starting with zero would not be a 5-digit number. After that, each of the other four digits can be selected in 3 ways. So the number of 5-digit numbers is 2*34 = 162
5040 numbers can be made.
120 5-digit numbers can be made with the numbers 12345.
9*10*10*10*10*10*10 or 9 million.