With $N=36$, $L=300$, $n_b=32$, $n_w=4$ and $c=50$, the 90th percentile of the distribution is:
A300
B310
C305
D32.4
Answer & Solution
Correct answer: C. 305
1. Use $P_k=L+\left(\dfrac{k\frac{N}{100}-n_b}{n_w}\right)c$ with $k=90$.
2. Position: $90\times\dfrac{36}{100}=32.4$.
3. Numerator: $32.4-32=0.4$.
4. $P_{90}=300+\dfrac{0.4}{4}\times 50=300+5=305$.
5. Reporting $32.4$ stops at the position; adding rather than subtracting $n_b$ pushes the answer past $310$.
_Source: NECTA ACSEE 2023 Advanced Mathematics 142/1, Question 4: Statistics_
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