The number of circular pipes with an inside diameter of 1 inch which will carry the same amount of water as a pipe with an inside diameter of 6 inches is :

Answer: Option C

Let the length of each pipe be $$l$$ inches
Then, volume of water in thinner pipe :
$$\begin{array}{rl}& =\left[\pi \times {\left(\frac{1}{2}\right)}^2\times 1\right]\text{cu.inch}\\ & =\left(\frac{\pi l}{4}\right)\text{cu.inch}\end{array}$$
Volume of water in thinker pipe :
$$\begin{array}{rl}& =\left(\pi \times 3^2\times l\right)\text{cu.inch}\\ & =\left(9\pi l\right)\text{cu.inch}\end{array}$$
∴ Required number of pipes :
$$\begin{array}{rl}& =\frac{9\pi l}{\left(\frac{\pi l}{4}\right)}\\ & =36\end{array}$$