3 blue + 3 yellow = 6 total marbles.
The probability of picking a blue one first would be 3/6 ( 3 blue out of 6 total marbles).
3/6 reduces to 1/2.
After picking the first marble, there would be 5 marbles left in the bag.
The probability of picking a yellow one would be 3/5 ( 3 yellow and 5 total marbles left).
The probability of picking a blue then a yellow then becomes:
1/2 x 3/5 = 3/10 = 0.30 = 30%