Let X be the random variable representing monthly trainee income. X is distributed with mean $1100 and standard deviation $150. You want to find the proportion of trainees that earn less than $900 per month, or Pr(X < 900).
Using normalcdf (on a TI calculator, for instance), you would compute
normalcdf(-1E99, 900, 1100, 150)
to get a proportion of approximately 0.09121, or 9.121%.
That is, the syntax for normalcdf is
normalcdf(lower limit, upper limit, mean, standard deviation)
In this case, you pick a very large negative number for "lower limit" in order to simulate negative infinity.
