To find the relative frequency of a non-defective unit, we first need to determine the number of non-defective units.
From the total of 500 calculator units, if 25 units have non-functioning keys (defective), we can find the number of non-defective units:
\[ \text{Number of non-defective units} = 500 - 25 = 475 \]
Now, the relative frequency of a non-defective unit is calculated as the ratio of non-defective units to the total units checked:
\[ \text{Relative frequency} = \frac{\text{Number of non-defective units}}{\text{Total number of units}} = \frac{475}{500} \]
Now, we simplify this fraction:
Both the numerator and the denominator 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 of items is:
\[ \frac{19}{20} \]
The final answer is:
19/20