x = hours of use, x normally distributed with mean = 20 and SD = 2.

want to find P(x < 18)

transform x to standard normal distribution using transformation z = x - mean / SD = x - 20 /2

P (x < 18) = P(z < 18-20/2) = P(z < -1) = 0.1587

P(x > 18) = 1 - P(x < 18) = 1 - 0.1587 = 0.8413

P(x > 25) = 1 - P(x < 25) = 1 - P(z < 25-20/2) = 1 - P(z < 2.5) = 1 - 0.9938 = 0.0062