To find the relative frequency of a non-defective unit from the batch of items, we start with the following information:
- Total calculator units checked = 500
- Non-functioning keys (defective units) = 25
First, we need to determine the number of non-defective units:
\[ \text{Non-defective units} = \text{Total units} - \text{Defective units} = 500 - 25 = 475 \]
Next, we calculate the relative frequency of a non-defective unit, which is given by the formula:
\[ \text{Relative frequency} = \frac{\text{Number of non-defective units}}{\text{Total units}} = \frac{475}{500} \]
Now we can simplify the fraction \(\frac{475}{500}\):
Both the numerator and 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:
\[ \boxed{\frac{19}{20}} \]