# copy original dataframe into a new one: my$\_$mtcars

# mtcars object is one of many built$-$in data sets in R$.$ So you do not need to worry about creating mtcars.

my$\_$mtcars $<-$ mtcars

# $1$: investigate my$\_$mtcars using str function. How many variables and observations are included in this dataframe?

# $2$: calculate engine displacement per cylinder and save it as a new variable 'UnitEngine' in the dataframe. Populate the two XXXX below

my$\_$mtcars$XXXX $<-$ my$\_$mtcars$XXXX$/$my$\_$mtcars$cyl

# $3.$ summarize the new variable 'UnitEngine': use summary function