The answer is 72
b/c 80% is equal to .80
so you do 90x.80
Given:
y is directly proportional to x.
when 
To find:
The equation that connects y with x.
Solution:
It is given that, y is directly proportional to x. So,

...(i)
Where, k is the constant of proportionality.
We have,
when
. Substituting these values in (i), we get
Divide both sides by 5.
Substituting
in (i), we get
Therefore, the required equation is
.
Given:
μ = 89 ng/ml, population mean
σ = 23 ng/ml, population standard deviation
Random variable, x = 100
To test P(x < 100), calculate the z-score.
z = (x-μ)/σ = (100 - 89)/23 = 0.4783
From standard tables, obtain
P(x < 100) = 0.6838 = 68.4%
Answer: 68.4%