7. Given (x+y) mod 5 =3 and (x-y) mod 2=0, find out a pair of x and y. (1%) 8. Does the set of residue (mod 5) form a group with respect to modular addition? Justify your answer. (1%) 9. Does the set of residue (mod 6) form a group with respect to modular multiplication? Justify your answer. (1%) 10. In Diffie-Hellman key exchange scheme, p=11, X=3, XB-4, find out the key. (1%)