Dplyr

Retrieve the Ozone column of the built-in airquality data frame. 

Retrieve the Month, Day and Ozone columns of the built-in airquality data frame in this order. 

Retrieve columns: mpg – cyl – hp – wt – gear from mtcars in this order for cars that weigh more than 3, and have 8 cylinders and 5 gears.
First retrieve the correct rows (cars) then retrieve the correct columns.
Remember to use as few blanks as possible. 

Retrieve columns: Ozone, Month and Day from airquality (in this order) for days that have missing Ozone concentrations.

Retrieve columns: Ozone, Month and Day from airquality (in this order) for days that do have Ozone concentrations.

Repeat the last mutate() from the demo but this time only show the first 6 rows of the result. 

Create a new data frame called DF: 
with columns: Ozone, Temp, Month, Day from airquality
and a new column: Temp_C with Temp in °C (°F-32)*5/9
without the months May (5) and June (6) and only for rows with Ozone concentrations

Calculate the mean weight for each group of cars that is defined by the cyl and am columns. 

Calculate the mean weight of cars with 4 gears for each group of cars defined by transmission type (am).

Group cars according to  number of cylinders
Calculate the mean and median weight per group of cars
Sort the result according to mean weight, put the largest first

Identify cars with 4 gears
Calculate mean weight per transmission type (am) and per motor type (vs)
Order the result according to the vs column, smallest values first

Use group_by() and summarize() to evaluate total ozone (sum) in each month.
Which month has the highest total ozone?

Retrieve the data for May (5) and June (6) from airquality.
Select the Ozone, Temp, Month and Day columns (in this order).
Order the rows according to the Temp column, lowest value first.

Sort the last counts table that was generated in the demo according to am, smallest value first.

Sort the last counts table that was generated in the demo according to the counts, smallest count first.

How many cars with 4 gears are there in mtcars? The answer must be an integer.

Calculate how many cars there are in mtcars per group of am for cars that have 4 gears
Sort the results according to count, highest count first.