To find the relative frequency of a non-defective unit from the batch of calculators, we first determine the number of non-defective units.
The total number of units checked is 500, and the number of non-functioning units is 25. Therefore, the number of non-defective (functioning) units is:
\[ 500 - 25 = 475 \]
Now, to express the relative frequency of a non-defective unit, we can write it as a fraction of the total checked units:
\[ \text{Relative frequency} = \frac{\text{Number of non-defective units}}{\text{Total units}} = \frac{475}{500} \]
Next, we can simplify the fraction \( \frac{475}{500} \). The greatest common divisor (GCD) of 475 and 500 is 25. So we divide both the numerator and the denominator by 25:
\[ \frac{475 \div 25}{500 \div 25} = \frac{19}{20} \]
Thus, the approximate relative frequency of a non-defective unit is:
\[ \boxed{\frac{19}{20}} \]