The accompanying data set contains three numerical variables, x₁, x₂, and x₃, and one categorical variable, x₄.

a. For x₄, how many of the observations are less than three?

b. Sort x₁, x₂, x₃, and then x₄ all in ascending order. After the variables have been sorted, what is the first observation for x₁, x₂, x₃, and x₄?

c. Sort x₁, x₂, x₃, and then x₄ all in descending order. After the variables have been sorted, what is the first observation for x₁, x₂, x₃, and x₄? 

d. How many missing values are there in x₁, x₂, x₃, and x₄? 

e. How many observations are there in each category in x₄?