12 men can do a piece of work in 24 days. How many days are needed to complete the work, if 8 men do this work ?

Answer: Option B

12 men can do a piece of work in 24 days
$$\Rightarrow {\text{M}}_1=12\text{ and }{\text{D}}_1=24$$
8 men can do this work in $${\text{D}}_2$$ days
$$\begin{array}{rl}& \Rightarrow {\text{M}}_2=8\\ & {\text{M}}_1{\text{D}}_1={\text{M}}_{\text{2}}{\text{D}}_{\text{2}}\\ & \Rightarrow 12\times 24=8\times {\text{D}}_2\\ & \Rightarrow {\text{D}}_2=\frac{12\times 24}{8}\\ & \Rightarrow {\text{D}}_2=36\text{ days}\end{array}$$