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Three positive integers are equally spaced on a number line. The middle number is $15$, and the largest number is $4$ times the smallest number. What is the smallest of these three numbers? (2022 AMC 8 Problems, Question \#6) 
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Let the smallest number be $x$. It follows that the largest number is $4 x$.  Since $x, 15$, and $4 x$ are equally spaced on a number line, we have  $$ \begin{aligned} 4 x-15 \& =15-x \textbackslash\textbackslash{} 5 x \& =30 \textbackslash\textbackslash{} x \& =(C) 6. \end{aligned} $$ 