Copy the first 10 countries for your top 20 countries file. Do not include the flags. Paste them into an excel spreadsheet. Include two rows at the bottom to calculate the total number population and area. Also include a column to the right calculating the population density for each country by dividing the population by the area. Please note you may need to modify the format of the numbers to include 2 decimal places.
Formula hint =c3/c4 or =sum(b1:b17)