10%
This is because the original probability of getting a green marble is 1/3. There are 7 green marbles out of 21 which simplifies to that ratio.
The second probability is 3/10 since there will be 6 green marbles left out of 20 total marbles. This simplifies to that ratio.
Now we multiply the two ratios together.
1/3 * 3/10 = 3/30 or 1/10, which is equal to 10%