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# Difference between revisions of "2012 AMC 10B Problems/Problem 6"

## Problem

In order to estimate the value of $x-y$ where $x$ and $y$ are real numbers with $x > y > 0$, Xiaoli rounded $x$ up by a small amount, rounded $y$ down by the same amount, and then subtracted her rounded values. Which of the following statements is necessarily correct?

$\textbf{(A)}$ Her estimate is larger than $x-y \qquad \textbf{(B)}$ Her estimate is smaller than $x-y \qquad \textbf{(C)}$ Her estimate equals $x-y \qquad \textbf{(D)}$ Her estimate equals $x-y \qquad \textbf{(E)}$ Her estimate is $0$

## Solution

Let's define $z$ as the amount rounded up by and down by.

The problem statement tells us that Xiaoli performed the following computation:

$\left(x+z\right) - \left(y-z\right) = x+z-y+z = x-y+2z$

We can see that $x-y+2z$ is greater than $x-y$, and so the answer is $\boxed{\textbf{(A)} \text{Her estimate is larger than} x-y}$.