Alter Digit

Let an integer to be called an alter integer if it satisfies the conditions below.

  1. When a certain one of its digits is deleted, the integer is reduced to one ninths of its original value.

  2. The resulting integer is divisible again by 9.

Find the sum of all alter integers that are not divisible by 10.


The answer is 170505.

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1 solution

Patrick Corn
Jan 8, 2019

The digit that is deleted must be a 0 0 or a 9 , 9, because the digit sum must be divisible by 9 both before and after the deletion.

If the digit is a 9 , 9, the situation is that A = 1 0 k + 1 x + 9 1 0 k + y , A = 10^{k+1} x + 9 \cdot 10^k + y, where y < 1 0 k , y < 10^k, and B = 1 0 k x + y B = 10^k x + y is the post-deletion integer, which is 1 / 9 1/9 of A . A. So we get 9 ( 1 0 k x + y ) = 1 0 k + 1 x + 9 1 0 k + y 8 y = 9 1 0 k + 1 0 k x \begin{aligned} 9(10^k x + y) &= 10^{k+1} x + 9 \cdot 10^k + y \\ 8y &= 9 \cdot 10^k + 10^k x \end{aligned} which is manifestly impossible, since the right side is less than 8 1 0 k . 8 \cdot 10^k.

If the digit is a 0 , 0, we similarly get A = 1 0 k + 1 x + y , A = 10^{k+1}x + y, B = 1 0 k x + y , B = 10^k x + y, and 9 ( 1 0 k x + y ) = 1 0 k + 1 x + y 8 y = 1 0 k x \begin{aligned} 9(10^k x + y) &= 10^{k+1} x + y \\ 8y &= 10^k x \end{aligned} Since y < 1 0 k , y < 10^k, we must have x < 8. x<8. So there are exactly 7 7 values of x x to consider. In each case, note that y y will be divisible by 5 , 5, so we'll need y y to be odd so that A A is not divisible by 10. 10.

For x = 1 x=1 we get y = 1 0 k / 8 y = 10^k/8 is an odd integer, so k = 3 , y = 125 , A = 10125 , B = 1125. k=3, y = 125, A = 10125, B = 1125.

For x = 2 x=2 we get y = 1 0 k / 4 y=10^k/4 is an odd integer, so k = 2 , y = 25 , A = 2025 , B = 225. k=2, y = 25, A = 2025, B = 225.

For x = 3 x=3 we get y = 3 1 0 k / 8 y = 3 \cdot 10^k/8 is an odd integer, so k = 3 , y = 375 , A = 30375 , B = 3375. k=3, y = 375, A = 30375, B = 3375.

For x = 4 x=4 we get y = 1 0 k / 2 y= 10^k/2 is an odd integer, so k = 1 , y = 5 , A = 405 , B = 45. k=1, y = 5, A = 405, B = 45.

For x = 5 x=5 we get y = 5 1 0 k / 8 y=5 \cdot 10^k/8 is an odd integer, so k = 3 , y = 625 , A = 50625 , B = 5625. k=3, y = 625, A = 50625, B = 5625.

For x = 6 x=6 we get y = 3 1 0 k / 4 y=3 \cdot 10^k/4 is an odd integer, so k = 2 , y = 75 , A = 6075 , B = 675. k=2, y = 75, A = 6075, B = 675.

For x = 7 x=7 we get y = 7 1 0 k / 8 y=7 \cdot 10^k/8 is an odd integer, so k = 3 , y = 875 , A = 70875 , B = 7875. k=3, y = 875, A = 70875, B = 7875.

The sum of all the A A 's is 170505 . \fbox{170505}.

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