I am not sure did i understand very well and i would like to someone check for me
15 . Return the name of each category with the total number of apps that belong to it
SELECT category, COUNT(*) FROM fake_apps GROUP BY category;
16 . Return the price and average number of downloads grouped by price.
SELECT price, cast(ROUND(AVG(downloads)) AS int) FROM fake_apps GROUP BY price;
18 . Return the name and category and price of the most expensive app for each category.
SELECT name, category, MAX(price) FROM fake_apps GROUP BY category;