The value of 46 is the mean, represented by a Z-score of 0, corresponding to 50%. 79 deviates from the mean by 79-46=33, that is, 3 standard deviations.
Therefore Z=3. Corresponding to 0.99865 or 99.865%.
So the range 46-79 corresponds to 50%-99.865%, a range of 49.865%, in other words, 49.865% of lightbulb replacement requests are in the range, 46-79.