Considering the lowest five digit hexadecimal number is 10000 (65,536) and the highest is FFFFF (1,048,575), there are 983,040 different hexadecimal numbers that are five digits.
12! / 9! / 3!
Two, 0 and 1.
There can be only one.
There are 27 of them.
How many strings of three digits are there? 000 to 999, or a total of 1000. How many strings of three digits contain the same three digits? That's 000, 111, 222 ... 999! ten in total. The difference is your answer: 1000-10 = 990.
-- 893 of them have no 4's. -- Three of them have exactly one 4: 4xx, x4x, and xx4. -- Three of them have exactly 2 4's: 44x, 4x4, and x44. -- One of them has exactly three 4's: 444. -- None has more than three 4's.
Assuming that 001, 080, etc are not allowed (that is a leading zero or two is not permitted), the smallest number with exactly three digits is 100. The largest number with exactly three digits is 999. So there are 999 - 100 + 1 = 900 numbers with exactly three digits.
The number of strings of four decimal digits that have exactly three digits which are nine is the number of ways to choose three places to choose the nines multiplied by the number of possibilities for the other digits (because you can put all of the possibilities in an array so that each column has a different way of arranging the nines and that each row has a different set of other digits (because they do not affect each other)). The number of ways to choose the three places to choose the nines is the same as the number of ways of choosing 4-3=1 spot for there not to be a nine. The number of ways of choosing 1 spot where there are 4 spots is 4 (the first, the second, the third, or the fourth). Since there are 9 digits that are not nine, the number of choices for the other digits is 9 (since there is only one digit). Thus, there are 4*9=36 strings of four decimal digits that have exactly three digits which are nine.
Only three groups: 44x, 4x4, and x44, where 'x' is not 4. yet you have to multiply each group 9 times because x could be 1,2,3...9,0 but not 4 so answer: 9*3
There are 5000 such strings.
Every number from 100 to 999 inclusive !