There are 16 total marbles with 6 blue ones.
Picking a blue one first is 6/16, reduces to 3/8
After picking a blue one first there are 5 blue ones left and 15 total marbles, so picking another blue would be 5/15, reduces to 1/3
The probability of picking both blue, multiply the two together:
3/8 x 1/3 = 3/24 = 1/8 = 0.125 x 100 = 12.5%