To find the relative frequency of a non-defective unit, we need to determine the ratio of non-defective units to the total number of units checked.
Here, it's stated that out of 500 calculator units, 25 of them have non-functioning keys (which we can assume represent defective units). Therefore, the number of non-defective units is:
\[ 500 - 25 = 475 \]
The relative frequency of a non-defective unit is calculated as:
\[ \text{Relative Frequency} = \frac{\text{Number of Non-defective Units}}{\text{Total Units}} = \frac{475}{500} \]
Now, to express this fraction in its simplest form:
The greatest common divisor (GCD) of 475 and 500 is 25. We can simplify the fraction by dividing both the numerator and the denominator 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 answer is B) 19/20.