Solution:
There were 30 marbles in the back of these 20 or blue and the rest are yellow
20 blue marbles
10 yellow marbles
a) For a game, each marble was chosen and 4/5 of the 20 blue marbles was chosen, i.e.

16 blue marbles was chosen
That means,

1/5 yellow marbles was chosen,

The number of bag's marble chosen is

Hence, the number of bag's marble chosen is 18
b) For the number of blue marbles not chosen

Hence, the number of blue marbles not chosen is 4
c) The number of yellow marbles chosen is 2