To find the relative frequency of a non-defective unit, we first need to determine the number of non-defective units in the batch.
There are a total of 500 calculator units, and 25 of them have non-functioning keys. Thus, the number of non-defective units is:
\[ 500 - 25 = 475 \]
Next, we can express the relative frequency of non-defective units as a fraction of the total number of units:
\[ \text{Relative frequency of non-defective units} = \frac{\text{Number of non-defective units}}{\text{Total number of units}} = \frac{475}{500} \]
Now, we simplify the fraction \( \frac{475}{500} \). Both the numerator and the denominator can be divided by 25:
\[ \frac{475 \div 25}{500 \div 25} = \frac{19}{20} \]
So the simplified form of the fraction representing the relative frequency of a non-defective unit is:
\[ \frac{19}{20} \]
Thus, the correct response is:
Start Fraction 19 over 20 End Fraction