If 10 men can do a piece of work in 12 days, the time taken by 12 men to do the same piece of work will be:

Answer: Option B

Here, we use work equivalence method;
10 × 12 = 12 × x
Or, x = 10 days

To understand the work equivalence method, we use a graphic as follows:

Men   Days
10 ↓    12
12     ↑ x (let)
Here, the two arrows, downward (↓) and upward (↑) show variation between men and days.
[If downward arrows show decrements then upward arrows show increments and vice-verse.]
Thus,
$$\begin{array}{rl}& \frac{10}{12}=\frac{\text{x}}{12}\\ & \text{or, x}=\frac{10\times 12}{12}\\ & =10\text{days}\end{array}$$