Explanation:

- Let's denote the original number of men as $x$.

- The work is initially estimated to take 10 days with $x$ men.

- The work actually finishes in 8 days because 5 more men joined, so the total number of men working becomes $x+5$.

- From the work/time formula, $x\times 10=(x+5)\times 8$.

- Solving the equation:

$$

10x = 8x + 40

$$

$$

2x = 40

$$

$$

x = 20

$$

- This means the original number of men is 20.

Option 3: 20

.