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?

Answer: Option D

Let the required number of bottles be x.
More weavers, More mats (Direct Proportion)
More days, More mats (Direct Proportion)
$$\begin{array}{c}\text{Wavers}4:8\text{hfill}\\ \text{Days}4:8\text{hfill}\end{array}\}::4:x$$
$$\begin{array}{rl}& \therefore 4\times 4\times x=8\times 8\times 4\\ & \Rightarrow x=\frac{8\times 8\times 4}{4\times 4}\\ & \Rightarrow x=16\end{array}$$