Question

The product of all digits of positive integer M is 105. How many such Ms are there with distinct digits?

Answer 1

Answer:

6

Step-by-step explanation:

105 is a composite number (not prime).  If you decompose 105 into its prime factors (see attached image), you get 105 = 3 x 5 x 7.  This means that the product 105 can only be written this way; any other way, for example 15 x 7 would not use numbers that are single digits.

The question boils down to how many ways the digits 3, 5, 7 can be arranged, using each digit exactly once.

That's a permutation problem, and it can be answered easily by thinking this way:  given the digits 3, 5, 7, there are 3 choices for the first digit, then 2 for the second digit (for EACH of the first choices), then only 1 choice for the final digit.

3 x 2 x 1 = 6 ways that 3, 5, 7 can be arranged to make a 3-digit number.  Here they are:

357, 375, 537, 573, 735, 753