If 7 maids with 7 mops cleaned 7 floors in 7 hours, how long would it take 3 maids to mop 3 floors with 3 mops ?

Answer: Option D

Since each maid would work with one mop,
So,we shall consider 1 maid and 1 mop as 1 unit
Let the required time be x hours
Less maids and mops, More time (Indirect proportion)
Less floor, Less time (Direct proportion)
\left. \begin{gathered} {\text{Maids & Mops 3}}:7 \hfill \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Floors 7}}:3 \hfill \\ \end{gathered} \right\}::7:x
$$\begin{array}{rl}& \therefore 3\times 7\times x=7\times 3\times 7\\ & \iff x=\frac{\left(7\times 3\times 7\right)}{\left(3\times 7\right)}\\ & \iff x=7\end{array}$$