If 7 spiders make 7 webs in 7 days, then 1 spider will make 1 web in how many days?

Answer: Option C

Let the required number days be x.
Less spiders, More days (Indirect Proportion)
Less webs, Less days (Direct Proportion)
$$\begin{array}{c}\text{Spiders}1:7\text{hfill}\\ \text{Webs}7:1\text{hfill}\end{array}\}::7:x$$
$$\begin{array}{rl}& \therefore 1\times 7\times x=7\times 1\times 7\\ & \Rightarrow x=7\end{array}$$