A number is called exuberant if it has the property that it is a multiple of its unit digit. How many exuberant 2-digit numbers are there?
Details and assumptions
The number is a 1-digit number.
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Let the number be 1 0 a + b , where 1 ≤ a ≤ 9 . Fix b , then we want 1 0 a to be a multiple of b .
Case 1: b = 1 . Then 1 0 a is a multiple of 1, so a is a multiple of 1 - 9 possibilities.
Case 2: b = 2 . Then 1 0 a is a multiple of 2, so a is a multiple of 1 - 9 possibilities.
Case 3: b = 3 . Then 1 0 a is a multiple of 3, so a is a multiple of 3 - 3 possibilities.
Case 4: b = 4 . Then 1 0 a is a multiple of 4, so a is a multiple of 2 - 4 possibilities.
Case 5: b = 5 . Then 1 0 a is a multiple of 5, so a is a multiple of 1 - 9 possibilities.
Case 6: b = 6 . Then 1 0 a is a multiple of 6, so a is a multiple of 3 - 3 possibilities.
Case 7: b = 7 . Then 1 0 a is a multiple of 7, so a is a multiple of 7 - 1 possibilities.
Case 8: b = 8 . Then 1 0 a is a multiple of 8, so a is a multiple of 4 - 2 possibilities.
Case 9: b = 9 . Then 1 0 a is a multiple of 9, so a is a multiple of 9 - 1 possibilities.
Case 9: b = 0 . Then 1 0 a is a multiple of 0, so a is a multiple of 0 - 0 possibilities.
By the rule of sum, there are 9 + 9 + 3 + 4 + 9 + 3 + 1 + 2 + 1 + 0 = 4 1 such numbers.