The distribution of the birth weights of babies delivered at 40 weeks is approximately normal with a mean of 3500g and standard deviation of 430g.
Calculate the upper quartile of birth weight. What is the lower quartile?
Upper quartile:
You need to solve Pr(X > x) = 0.25 for x.
To do this, first find the value of z* such that Pr(Z > z*) = 0.25.
Now note that $\displaystyle z* = \frac{x - 3500}{430}$ and solve for x.
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Lower quartile is done the same way.