If 20 men working 5 hours a day complete a work in 4 days. Then in how many days will 10 men working 10 hours a day finish the same work? Assume that all men work at same efficiency.

Correct option is C

Given:
20 men working 5 hours a day complete a work in 4 days.
All men work at same efficiency.
Formula Used:
​$$D_1 × T_1 × M_1 = D_2 × T_2 × M_2$$​​
Solution:
​$$\text{Here, } D_1 = 4 \text{ days} \\T_1 = 5 \text{ hours} \\M_1 = 20 \text{ men} \\T_2 = 10 \text{ hours} \\\text{and } M_2 = 10 \text{ men}$$​​
Putting this values in the above formula get,
​$$4 \times 5 \times 20 = D_2 \times 10 \times 10 \\\Rightarrow D_2 = \frac{400}{100} \\\Rightarrow D_2 = 4$$​​