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Sagot :
Solution :
The test is distributed normally with mean of 72.8 and the standard deviation of 7.3
Finding numerical limits for the D grade.
D grade : Scores below the top 80% and above the bottom 10%.
Let the bottom limit for D grade be [tex]$D_1$[/tex] and the top limit for D grade be [tex]$D_2$[/tex].
First find the bottom numerical limit for a D grade is :
[tex]$P(X<D_1)= 0.10$[/tex]
[tex]$P(X\leq D_1)= 0.10$[/tex]
[tex]$P\left(\frac{X-\mu}{\sigma} \leq \frac{D_1-\mu}{\sigma}\right) = 0.10$[/tex]
[tex]$P\left(Z \leq \frac{D_1-72.8}{7.3}\right) = 0.10$[/tex] Â Â ..........(1)
From (1)
[tex]$\frac{D_1 - 72.8}{7.3} = -1.28$[/tex]
[tex]$D_1 = -1.28(7.3)+72.8$[/tex]
   = 63.45
    ≈ 64
Now the top numerical limit for D grade :
[tex]$P(X>D_2)= 0.80$[/tex]
[tex]$1-P(X\leq D_2)= 0.80$[/tex]
[tex]$P(X\leq D_2)= 1-0.80$[/tex]
[tex]$P(X\leq D_2)= 0.20$[/tex]
[tex]$P\left(\frac{X-\mu}{\sigma} \leq \frac{D_2-\mu}{\sigma}\right) = 0.20$[/tex]
[tex]$P\left(Z \leq \frac{D_2-72.8}{7.3}\right) = 0.20$[/tex] Â Â ..........(2)
From (2)
[tex]$\frac{D_2- 72.8}{7.3} = -0.84$[/tex]
[tex]$D_12= -0.84(7.3)+72.8$[/tex]
   = 66.668
    ≈ 67
Therefore, the numerical limit for a D grade is 64 to 67.
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