$12.$ Write a function called "add$\_$four$("$ that adds the number $4$ to each element of a numeric vector. $(2$pt$)$

$13.$ Write a function called $"$my$\_$fun$()"$ that divides each element of a numeric vector by the sum of that vector. $(2$pt$)$

$14.$ Write a for loop that calculates the mean of each of the first three variables in the data frame trees $($accessible in base R$)$ and saves them to an output vector. $(4$pts$)$ This is multipart:

$-$ create empty output vector $(2$pt$)$

$-$ write for loop that fills the empty vector with calculated means $(2$pt$)$