The solution to the questions are
Generally, the equation for the mean is  mathematically given as
The following is the formula for determining the X score:
X = Mean + Z*SD
Part a
The value of 0.841621 is assigned as the Z-score for the top 20%. (using excel)
X = 315 + 0.841621*16
X= 328.4659
Part b
The Z-scores range from -0.84162 to 0.841621 for the middle 60 percent of the population (using excel)
X '= 315 - 0.841621*16
X'= 301.5341
X" = 315 + 0.841621*16
X"= 328.4659
c)
The value of -0.84162 is assigned as the Z score for the top 80 percent (by using excel)
X''' = 315 - 0.841621*16
X'''= 301.5341
d)
The Z score for the group comprising the worst 15 percent is written as -1.03643. (by using excel)
X'''' = 315 – 1.03643*16
X''''= 298.4171
In conclusion,
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