Though by stepwise solving this question i got the answer as 2/5 but still not convinced i that the probability will remain the same regardless of the sequence of the draw and why its going to be constant across any draw as ratio of blue marbles to the total marble.
Try with a smaller example, or just calculate this explicitly.
I did and i got that (mentioned that in the question), wanted to understand the logic behind that, why? thanks!
See https://www.reddit.com/r/GRE/comments/1eaxk68/i_disagree_with_gregs_answer_please_help/ - that person has explained it quite well.