4 mat-weavers can weave 4 mats in 4 days. At the same rate, how many mats would be woven by 8 mat-weavers in 8 days?

Let the required number of bottles be x.
More weavers, More mats (Direct Proportion)
More days, More mats (Direct Proportion)
\[\left. \begin{gathered} {\text{Wavers}}\,\,\,\,4:8 \hfill \\ \,\,\,\,\,\,{\text{Days}}\,\,\,\,4:8 \hfill \\ \end{gathered} \right\}::4:x\]
$$\eqalign{ & \therefore 4 \times 4 \times x = 8 \times 8 \times 4 \cr & \Rightarrow x = \frac{{ {8 \times 8 \times 4} }}{{4 \times 4}} \cr & \Rightarrow x = 16 \cr} $$