So for the first one, you'll need to write out all the factors of both 30 and 16. (30--1, 2, 3, 5, 6, 10, 15, 30) (16--1, 2, 4, 8, 16) The biggest number that goes into both 30 and 16 is 2.
For the second kne, its the same thing. (48--1, 2, 3, 4, 6, 8, 12, 16, 24, 48) (18--1, 2, 3, 6, 9, 18) The biggest number that can go into both 48 and 18 is 6.