If the time taken to do a piece of work is inversely proportional to the number of men employed, we can use the formula:

$$T_1\times M_1=T_2\times M_2$$

Where:
- $T_1$ is the time taken by the first group of men,
- $M_1$ is the number of men in the first group,
- $T_2$ is the time taken by the second group of men,
- $M_2$ is the number of men in the second group.

Given:
- $T_1=5$ days,
- $M_1=45$ men,
- $M_2=25$ men.

We need to find $T_2$.

Using the formula:

$$5\times 45=T_2\times 25$$

$$225=25\times T_2$$

Solve for $T_2$:

$$T_2=\frac{225}{25}=9\text{ days}$$