Let x and y denote the thickness of two different layers of ink deposited onto a substrate. X is normally…

Let x and y denote the thickness of two different layers of ink deposited onto a substrate. X is normally distributed with mean 0.1 mm and standard deviation 0.00031mm, and y distributed with mean 0.23 mm and standard deviation 0.00017mm, assume x and y are independent, why is the probability that the total ink thickness is less than 0.3337?