Here it goes:
a. Because you have four blue marbles out of the 12 total marbles, the probability is 4/12. YOU MUST SIMPLIFY down to 1/3.
b. There are 12 marbles in the bag right now, and the probability is 4/12, you must add some marbles. 1/10 multiplied by 4 is 4/40. There are 4 blue marbles right now, and 40 minus the 12 there are right now is 38, so you would need to add 38 non-blue marbles.