If 16 men or 20 women can do a piece of work in 25 days. In what time will 28 men and 15 women do it ?

Answer: Option D

If number of days at work is same
$$\begin{array}{rl}& \text{So, we can equate directly}\\ & \text{16 men}=\text{20 women}\\ & \text{4 men}=\text{5 women}\\ & \text{5 women}=\text{4 men}\\ & \text{1 women}=\frac{4}{5}\text{men}\\ & \text{15 women}=\frac{4}{5}\times \text{15}\\ & \text{15 women}=\text{12 men}\end{array}$$
⇒ 16 men complete a work in 25 days
1 men complete a work in 400 days
$$\begin{array}{rl}& 28\text{ men}+\text{15 women}=?\\ & \downarrow \\ & 28\text{ men}+\text{12 men}=\text{40 men}\\ & \text{work in }=\frac{25\times 16}{40}\\ & =\text{10 days}\\ & \\ & \mathbf{Alternate}:\\ & \text{16 m or 20 w}\to \text{25 days}\\ & \text{28 m and 15 w}\to \text{?}\\ & \frac{16\times 20\times 25}{\left(16\times 15\right)+\left(20\times 28\right)}\\ & =10\text{ days}\end{array}$$