1. Let U = {a, b, c}. Let S1 be the set of strings over U whose first two letters are the same, and let S2 be the set of strings over U with length 3. Write down all the elements of S1 S2.

2. Let S1 be the set of all prime numbers, and S2 = {x | x N and x | 30}. Write down all the elements of S2\S1.

3. Let U = {a, b}. Let S1 be the set of strings over U that start and end with different letters, and let S2 be the set of strings over U with length 3. Write down all the elements of S2\S1.