# Difference between revisions of "2010 AMC 10B Problems/Problem 14"

## Problem

The average of the numbers $1, 2, 3,\cdots, 98, 99,$ and $x$ is $100x$. What is $x$?

$\textbf{(A)}\ \dfrac{49}{101} \qquad \textbf{(B)}\ \dfrac{50}{101} \qquad \textbf{(C)}\ \dfrac{1}{2} \qquad \textbf{(D)}\ \dfrac{51}{101} \qquad \textbf{(E)}\ \dfrac{50}{99}$

## Solution

We must find the average of the numbers from $1$ to $99$ and $x$ in terms of $x$. The sum of all these terms is $\frac{99(100)}{2}+x=99(50)+x$. We must divide this by the total number of terms, which is $100$. We get: $\frac{99(50)+x}{100}$. This is equal to $100x$, as stated in the problem. We have: $\frac{99(50)+x}{100}=100x$. We can now cross multiply. This gives:

\begin{align*} 100(100x)=99(50)+x&\\ 10000x=99(50)+x&\\ 9999x=99(50)&\\ 101x=50&\\ x=\boxed{\textbf{(B)}\ \frac{50}{101}}&\\ \end{align*} (Error compiling LaTeX. ! LaTeX Error: Bad math environment delimiter.)