Answer:
z = 1.28 < a - 200/20
And if we solve for a we got
a = 200 + 1.28 * 20 = 225.6
So the value of height that separates the bottom 90% of data from the top 10% is 225.6.
Step-by-step explanation:
Let X the random variable that represent the scores of a population, and for this case we know the distribution for X is given by:
X ~ N (200,20)
For u = 200 and o = 20
For this case we can use the z score in order to solve this problem, given by this formula:
Z = x-u/o
For this part we want to find a value a, such that we satisfy this condition:
P (X > a) = 0.1 (a)
P (X < a) = 0.9 (b)
Both conditions are equivalent on this case. We can use the z score again in order to find the value a.
As we can see on the figure attached the z value that satisfy the condition with 0.9 of the area on the left and 0.1 of the area on the right it's z=1.28. On this case P(Z<1.28)=0.9 and P(z>1.28)=0.1
If we use condition (b) from previous we have this:
P ( X < a) = P (X-u/o < a - u/o) = 0.9
P (z < a-u/o) = 0.9
But we know which value of z satisfy the previous equation so then we can do this:
z = 1.28 < a - 200/20
And if we solve for a we got
a = 200 + 1.28 * 20 = 225.6
So the value of height that separates the bottom 90% of data from the top 10% is 225.6.
