To answer this question, first let us define that probability is:
Probability = number of favorable outcomes / total number of outcomes
In this case we want to select 124.1V to 125V so:
number of favorable outcomes = 10
Since the total range is between 123.0V to 125.0V:
total number of outcomes = 20
Therefore:
Probability = 10/20
Probability = 0.50
This means that 50% of the time, the voltage will be greater than 124.0V