Answer:
A baby that is 18 inches long receives a score of -2.
Explanation:
We are given the following information in the question:
The doctor uses 20 inches as a reference point.
Mean, μ = 20 inches
We assume that the distribution of heights of newborn babies is a bell shaped distribution that is a normal distribution.
Formula:
![z_(score) = \displaystyle(x-\mu)/(\sigma)](https://img.qammunity.org/2020/formulas/mathematics/college/5bpvqdbyqd8y38zhlcp80hz1p4ka5nivnl.png)
A baby that is 21 inches long receives a score of +1.
Thus, we can write:
![1 = \displaystyle(21-20)/(\sigma)\\\\\sigma = 1](https://img.qammunity.org/2020/formulas/mathematics/high-school/8hhjrboyre7576u764r7y6ih2lvxghf4eh.png)
We have to find z-score for baby that is 18 inches long.
Putting x = 18, we get:
![z_(score) = \displaystyle(18-20)/(1) = -2](https://img.qammunity.org/2020/formulas/mathematics/high-school/o2pu2godlzfmo6meq5afeg4xg9lbygk7en.png)
A baby that is 18 inches long receives a score of -2.