Help with SELECT queries
Your answer to number 5 is lacking a GROUP BY (corresponding total quantity).

If your course requires you to return sets (as opposed to multi-sets) you may need to eliminate potential duplicates from some of those queries. Your results may not show duplication now, due to the data in the tables, but to be correct for different states of the database (different contents) you may need to add DISTINCT, unless constraints exist to make it unnecessary.

Some of your queries could equally be expressed using joins - if you have covered those, it might be a good idea to use some.