removed homework 1
finished problem 1
finished problem 2
finished problem 3
finished problem 4
finished problem 5
finished problem 6
problem 7 solution without correlated subquery
problem 7 solution with subqueries
finished problem 8
finshed problem 1
finished problem 2
finished problem 3
finished problem 4
finished problem 5
finished problem 6
finished problem 7
finished problem 8
finished problem 9
finished problem 1