45 men can complete a piece of work in 16 days. Four days they started working , 36 more men joined them. How many days will they take to complete the remaining work ?

Answer: Option C

According to the question,
Let the remaining work is complete in D days
$$\text{4}{\text{5}}_{\text{men}}\times \text{1}{\text{6}}_{\text{days}}$$    = $$\left(\text{4}{\text{5}}_{\text{men}}\times {\text{4}}_{\text{days}}\right)$$    + $${\left(\text{45}+\text{36}\right)}_{\text{men}}$$   $$\times \text{D}$$
$$\begin{array}{rl}& \Rightarrow \frac{540}{81}=\text{D}\\ & \Rightarrow \text{D}=6\frac{2}{3}\text{ days}\end{array}$$