Hi
Below: z = 0, z = ± 1, z= ±2 , z= ±3 are plotted.
Note: z = 0 (x value the mean) 50% of the area under the curve is to the left
and %50 to the right

Population: mean = 23, SD = 1
P(x < 21), z = -2
P(z < -2) = normalcdf( -10, -2) = .0228 TI syntax is normalcdf(smaller z, larger z) -10 a placeholder
P( 21 ≤ x ≤ 23) = .5-.0228 = .4772 0r 47.72%
Note: P(x ≤ 23) = .5
(50% of the Area under the Standard Normal curve is to the left of the mean)