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jmc

prealgebra senior

Problem

A number is called a visible factor number if it is divisible by each of its non-zero digits. For example, 102 is divisible by 1 and 2, so it is a visible factor number. How many visible factor numbers are there from 100 through 150, inclusive?
Solution
To begin with, note that all the numbers in question have a 1 in the hundreds place, and every number is divisible by 1, so we do not need to check it. So we need to see under what circumstances the number is divisible by its tens and units digits.

Let the three-digit number be We can then divide into cases, based on the digit

Case 1: .

We are looking for three-digit numbers of the form that are divisible by or where If is divisible by then 100 is divisible by Thus, the possible values of are 0, 1, 2, 4, and 5.

Case 2: .

We are looking for three-digit numbers of the form that are divisible by or where If is divisible by then 110 is divisible by Thus, the possible values of are 0, 1, 2, and 5.

Case 3: .

We are looking for three-digit numbers of the form that are divisible by or where If is divisible by then 120 is divisible by Also, must be divisible by 2, which means is even. Thus, the possible values of are 0, 2, 4, 6, and 8.

Case 4: .

We are looking for three-digit numbers of the form that are divisible by or where If is divisible by then 130 is divisible by Also, must be divisible by 3. Thus, the possible values of are 2 and 5.

Case 5: .

We are looking for three-digit numbers of the form that are divisible by or where If is divisible by then 140 is divisible by Also, must be divisible by 4. Thus, the possible values of are 0 and 4.

Case 6: .

Since the three-digit number must be between 100 and 150, the only number in this case is 150.

Adding up the possibilities gives us possible three-digit numbers.

Final answer
19