25 persons, each working 10 hours a day, can complete 2 units of work in 4 days. How many days will it take for 10 persons working 9 hours a day to complete 18 units of work?

Correct option is C

Given:
​$$M_1 = 25, H_1 = 10, W_1 = 2, D_1 = 4 \\M_2 = 10, H_2 = 9, W_2 = 18$$​​
Formula Used:
​$$\frac{M_1 D_1 H_1}{W_1} = \frac{M_2 D_2 H_2}{W_2}$$​​
Solution:
Substitute the values into the chain rule formula:
​$$\frac{25 \times 4 \times 10}{2} = \frac{10 \times D_2 \times 9}{18}$$​​
Simplify both sides:
​$$\frac{1000}{2} = \frac{90 \times D_2}{18} \\500 = 5 \times D_2 \\D_2 = \frac{500}{5} \\D_2 = 100 \text{ days}$$​​
So the correct answer is (c)