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 \(T_2\):

\[
T_2 = \frac{225}{25} = 9 \text{ days}
\]