方先生买了一盒马克笔。其中 $\frac{1}{7}$ 的马克笔是黑色的。其余马克笔的 $\frac{1}{3}$ 是红色的，剩下的都是绿色的。如果绿色马克笔有 400 支，方先生一共买了几支马克笔？ 答：___

1. Find the fraction of the remaining markers after black markers are accounted for

Fraction of remaining markers = $1 - \frac{1}{7} = \frac{6}{7}$

2. Find the fraction of the total markers that are green

Fraction of red markers = $\frac{1}{3} \times \frac{6}{7} = \frac{2}{7}$
Fraction of green markers = $\frac{6}{7} - \frac{2}{7} = \frac{4}{7}$

3. Equate the fraction of green markers to the actual quantity to find the total number of markers

$\frac{4}{7}$ of total markers = $400$
$4$ units = $400$
$1$ unit = $100$
Total markers ($7$ units) = $7 \times 100 = 700$