Change Rearrangement Puzzle
Lupe went to the store and paid for her purchase with a $ \ $ 10$ bill. She found that the digits making the amount of her purchase could be rearranged to make the amount she received back in change. If her purchase amount and her change amount were different and each amount was at least $\$ 1 $, how many possible amounts of change could she have received?
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
First, for simplicity, let's make all the amounts of money into integers by considering them all in cents. For example, $ \ $5.43$ becomes 543. Let the purchase price be $A=A_1A_2A_3$ and the amount of change be $B_1B_2B_3$ where $A_1$ represents the first digit of $A$ , $B_1$ represents the first digit of $B$ , $A_2$ represents the second digit of $A$ , etc.
We know that $A+B=1000$ , and we can conclude that $A_1+B_1=9$ because if $A_1+B_1<9$ then $A+B<1000$ and if $A_1+B_1=10$ then $A_2=B_2=A_3=B_3=0$ , but then the only way that B can be a rearrangement of the digits of A is if $A_1=B_1=5$ , which means $A=B=500$ , but the problem states that the price and the amount of change are different.
Since 9 is odd, we can also conclude that $A_1$ and $B_1$ are distinct, which, using the fact that $A$ 's digits can be rearranged to get B's digits, implies that $A_1=B_2$ or $A_1=B_3$ and $B_1=A_2$ or $B_1=A_3$ . We can also observe that A and B have the same remainder when divided by 9 because the remainder when $n$ is divided by 9 is equal to the remainder when the sum of the digits of $n$ is divided by 9 for all $n$ and the sum of the digits of A is obviously equal to the sum of the digits of B.
Since the remainder when 1000 is divided by 9 is 1, we can in fact conclude that the remainder when A and B are divided by 9 (and when the sum of their digits is divided by 9) is 5. Keeping in mind that two of the digits of $A$ are $A_1$ and $B_1$ and that $A_1+B_1=9$ , we can conclude that the other digit is 5, which is the only digit that would result in the sum having a remainder of 5 when divided by 9. By similar logic we can conclude that 5 is also one of the digits of $B$ . A little thought makes it clear that at least one of these 5's appears as the last digit in its number (that is, $A_3=5$ or $B_3=5$ ) since if neither of them appears as the last digit in a number, then $A_1=B_3$ and $B_1=A_3$ and $A_3+B_3=9\Rightarrow A+B$ ends in a 9, which is a contradiction. But if $A_3=5$ then the only way for the sum of $A$ and $B$ to end in a 0 is for $B_3=5$ , so we can conclude that $A_3=B_3=5$ , $A_1=B_2$ , and $A_2=B_1$ . So once we have picked a value for $A_1$ , the other 5 digits are all determined. Since both amounts are greater than a dollar, we know that $A_1$ can be any number between 1 and 8 for a total of 8 possible prices (and thus 8 possible amounts of change). To double check, we can work out $A$ and $B$ for each value of $A_1$ and reconvert them to dollars to make sure that the price and the amount of change satisfy the given conditions.
$A_1=1\Rightarrow A=\$1.85, B=\$8.15$ ;
$A_1=2\Rightarrow A=\$2.75, B=\$7.25$ ;
$A_1=3\Rightarrow A=\$3.65, B=\$6.35$ ;
$A_1=4\Rightarrow A=\$4.55, B=\$5.45$ ;
$A_1=5\Rightarrow A=\$5.45, B=\$4.55$ ;
$A_1=6\Rightarrow A=\$6.35, B=\$3.65$ ;
$A_1=7\Rightarrow A=\$7.25, B=\$2.75$ ; and finally
$A_1=8\Rightarrow A=\$8.15, B=\$1.85$.
This confirms that there are $\boxed{8}$ possible amounts of change.