To determine the relative frequency of a non-defective unit from the batch of 500 calculators, we have the following information:
- Total units checked: 500
- Non-functioning units: 25
First, we need to find the number of non-defective units:
\[ \text{Number of non-defective units} = \text{Total units} - \text{Non-functioning units} = 500 - 25 = 475 \]
Now, we can calculate the relative frequency of a non-defective unit by dividing the number of non-defective units by the total number of units:
\[ \text{Relative frequency of non-defective units} = \frac{475}{500} \]
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 is:
\[ \frac{19}{20} \]
So, the correct answer is: \[ \text{Start Fraction } 19 \text{ over } 20 \text{ End Fraction} \]