The average number of hours that a kid spends per day working at a computer 3.5 hours. This distribution is normally distributed with the STDV of 0.9 hours. What percentage of kids spend more than 3.8 hours per day working at a computer?
Sanity check: Average is 3.5h, so 50% of kids spend less than 3.5h, 50% spend more (with Gaussian distribution mean and median are the same). That means that more than 50% of kids will spend LESS than 3.8h, and less than 50% of kids will spend MORE than 3.8h.
The term A=0.5*erfc() is the cumulative probability yp to that point (from -infinity). Since we want to know the probability from the point to +infinity, we must take 1-A.