To simplify the fraction \( \frac{25}{40} \), we can find the greatest common divisor (GCD) of the numerator and the denominator.
The GCD of 25 and 40 is 5.
Now, we can divide both the numerator and the denominator by their GCD:
\[ \frac{25 \div 5}{40 \div 5} = \frac{5}{8} \]
Thus, \( \frac{25}{40} \) in simplest form is \( \frac{5}{8} \).