Quiz Sorting not showing the correct answer

In quiz on sorting one of the question is not showing the correct response . That question appeared in some gate exam and according to its solution my answer is correct.
The question is “Assume that a merge sort algorithm in the worst case takes 30 seconds for an input of size 64. Which of the following most closely approximates the maximum input size of a problem that can be solved in 6 minutes?”

i think n=512 should be correct as worst case or best case complexity is same in mergesort (nlogn)

But its showing my response is incorrect and at the same time not showing what the correst answer is?
I checked it on google . This question has appeard in GATE exam and 512 is the correct answer.

yaa so you can contact [email protected] for this issue…but your ans is correct…

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I mailed [email protected] regarding my query but they didn’t responded .