Exuberant Numbers

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 09 = 9 09=9 is a 1-digit number.


The answer is 41.

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1 solution

Arron Kau Staff
May 13, 2014

Let the number be 10 a + b 10a + b , where 1 a 9 1 \leq a \leq 9 . Fix b b , then we want 10 a 10a to be a multiple of b b .

Case 1: b = 1 b=1 . Then 10 a 10a is a multiple of 1, so a a is a multiple of 1 - 9 possibilities.

Case 2: b = 2 b=2 . Then 10 a 10a is a multiple of 2, so a a is a multiple of 1 - 9 possibilities.

Case 3: b = 3 b=3 . Then 10 a 10a is a multiple of 3, so a a is a multiple of 3 - 3 possibilities.

Case 4: b = 4 b=4 . Then 10 a 10a is a multiple of 4, so a a is a multiple of 2 - 4 possibilities.

Case 5: b = 5 b=5 . Then 10 a 10a is a multiple of 5, so a a is a multiple of 1 - 9 possibilities.

Case 6: b = 6 b=6 . Then 10 a 10a is a multiple of 6, so a a is a multiple of 3 - 3 possibilities.

Case 7: b = 7 b=7 . Then 10 a 10a is a multiple of 7, so a a is a multiple of 7 - 1 possibilities.

Case 8: b = 8 b=8 . Then 10 a 10a is a multiple of 8, so a a is a multiple of 4 - 2 possibilities.

Case 9: b = 9 b=9 . Then 10 a 10a is a multiple of 9, so a a is a multiple of 9 - 1 possibilities.

Case 9: b = 0 b=0 . Then 10 a 10a is a multiple of 0, so a 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 = 41 9 + 9+3+4+9+3+1+2+1 + 0= 41 such numbers.

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