The strength of a standard structural steel for a certain construction project follows a normal distribution with mean 20 N/mm2 and variance 16 N/mm2. Find the probability that a randomly selected specimen from a supply batch of this steel has strength less than 25 N/mm2.
(a) Use the integral of the probability mass function,
(b) transform the normal distribution into a standard normal distribution and use the integral of the probability mass function for that distribution.
(c) Use the statistical tables for the cumulative standard normal distribution.