To find the relative frequency of a non-defective unit, we first need to determine the number of non-defective units.
The total number of units is 500, and the number of non-functioning keys (defective units) is 25. Therefore, the number of non-defective units is:
\[ 500 - 25 = 475 \]
Now, the relative frequency of a non-defective unit can be expressed as:
\[ \text{Relative frequency} = \frac{\text{Number of non-defective units}}{\text{Total number of units}} = \frac{475}{500} \]
Next, we can simplify this fraction. Both 475 and 500 can be divided by 25:
\[ \frac{475 \div 25}{500 \div 25} = \frac{19}{20} \]
Thus, the relative frequency of a non-defective unit from this batch is:
\[ \frac{19}{20} \]
Therefore, the answer is:
19/20