Answer:
[tex]\large \boxed{\text{15 min}}[/tex]
Step-by-step explanation:
The cars are diverging at a combined speed of 60 + 40 =100 mi/h.
That is the same as if one car stood still and the other sped away at 100 mi/h.
[tex]\text{Time} = \text{25 mi} \times \dfrac{\text{1 h}}{\text{100 mi}} = \text{0.25 h} = \text{15 min}\\\text{The cars will be 25 mi apart in $\large \boxed{\textbf{15 min}}$}[/tex]
