jmc

The number of men and the amount of time to dig the foundation are inversely proportional. Let $m$ equal the number of men and $d$ equal the number of days to complete the foundation. This implies that $md=k$ for some constant $k$. From the given information, $15\cdot 4=60=k$. Knowing the value of $k$ we can solve for the number of days it would have have taken 25 men to dig the foundation: $$\begin{array}{rl}25\cdot d& =60\\ \Rightarrow d& =60/25=12/5=\boxed{2.4}\end{array}$$