Question

How many integers between 100 and 999 (inclusive) have distinct digits

Answer 1

Answer:

648

Step-by-step explanation:

Running this in Python, with the code as follows,

import math

cur_numbers = [0] * 3

num = 0

for i in range(100, 1000):

   cur_numbers[2] = i % 10

   i = math.floor(i/10)

   cur_numbers[1] = i % 10

   i = math.floor(i/10)

   cur_numbers[0] = i % 10

   if(len(set(cur_numbers)) == 3):

       num += 1

       print(cur_numbers)

       

print(num), we get 648 as our answer.

Another way to solve this is as follows:

There are 9 possibilities for the hundreds digit (1-9). Then, there are 10 possibilities for the tens digit, but we subtract 1 because it can't be the 1 same digit as the hundreds digit. For the ones digit, there are 10 possibilities, but we subtract 1 because it can't be the same as the hundreds digit and another 1 because it can't be the same as the tens digit. Multiplying these out, we have

9 possibilities for the hundreds digit x 9 possibilities for the tens digit x 8 possibilities for the ones digit  = 648