By using a common denominator of 21, we can place equally spaced tick marks at intervals of \( \frac{1}{21} \). This allows us to plot \( \frac{1}{3} \) as \( \frac{7}{21} \) without first plotting 1. The number line will have tick marks at \( \frac{1}{21}, \frac{2}{21}, \ldots, \frac{21}{21} \), with \( \frac{2}{7} \) at \( \frac{6}{21} \) and \( \frac{1}{3} \) at \( \frac{7}{21} \).