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 mats be x
More weavers , More mates ( Direct proportion )
More days, More mats ( Direct proportion )
$$\begin{array}{c}\text{Weavers }4:8\text{hfill}\\ \text{Days }4:8\text{hfill}\end{array}\}::4:x$$
$$\begin{array}{rl}& \therefore 4\times 4\times x=8\times 8\times 4\\ & \iff x=\frac{\left(8\times 8\times 4\right)}{\left(4\times 4\right)}\\ & \iff x=16\end{array}$$