Even more aliasing
Let's practice your newfound aliasing skills some more before moving on!

Recall: SQL assumes that if you divide an integer by an integer, you want to get an integer back.

This means that the following will erroneously result in 400.0:

SELECT 45 / 10 * 100.0;
This is because 45 / 10 evaluates to an integer (4), and not a decimal number like we would expect.

So when you're dividing make sure at least one of your numbers has a decimal place:

SELECT 45 * 100.0 / 10;
The above now gives the correct answer of 450.0 since the numerator (45 * 100.0) of the division is now a decimal!

Get the percentage of people who are no longer alive. Alias the result as percentage_dead. Remember to use 100.0 and not 100!:
-- get the count(deathdate) and multiply by 100.0
-- then divide by count(*)
SELECT COUNT (deathdate) * 100.0/ COUNT (*) AS percentage_dead
FROM people;

Get the number of years between the newest film and oldest film. Alias the result as difference:
-- get the count(deathdate) and multiply by 100.0
-- then divide by count(*)
SELECT MAX (release_year) - MIN (release_year) AS difference
FROM films;

Get the number of decades the films table covers. Alias the result as number_of_decades.
The top half of your fraction should be enclosed in parentheses:
SELECT (MAX(release_year) - MIN(release_year)) / 10.0
AS number_of_decades
FROM films