Runtime Calculation Sort-Algortihm I'm really struggling with the following exercise and I would really appreciate your help:
I have to calculate the expected runtime of a sort Algorithm with the following variants:

Variant   Runtime Behavior
1            n(n-1)
2              n
3            n log(n)


Given is, that the runtime for variant 1 is 1000 elements(n) is 20ms.
With the calculation for the other elements from variant 1, I have no problem because it's O(n^2) but I really don't know how to calculate the values for variant 2 &amp; 3.

Variant    Length n    expected runtime in ms
   1        500          20ms * 0.5^2 = 5ms
   1        4'000        20ms * 4^2   = 320ms
   2        2'000'000    ?
   3        4'096        ?


Thanks for your help in advance

Kind Regards
Do not even mention big-O here because that is for asymptotic behaviour and f(1000) doesn’t give you the slightest clue what f(2000000) would be. 

You are given that the runtime is f(n) = c x g(n) for three different functions c. Given the value of c x g(1000) for the first function g, you calculate c. Then you substitute c and n for all the cases.