Since nothing is mentioned, I would assume Hierarchical access.
Tavg = H1T1 + (1-H1)H2(T1+T2) + (1-H1)(1-H2)(T1+T2+T3)
Since there are 3 levels.
This formula can be interpreted as
[(Hit in Level 1) * (Time to access in level 1)] + [(Miss in Level 1) * (Hit in Level 2) * (Time to access Level 1 memory + Time to access Level 2 memory)] + [(Miss in Level 1) * (Miss in Level 2) * (Time to access Level 1 memory + Time to access Level 2 memory + Time to access Level 3 memory)]
In the last term, H3 is assumed to be 1 as the word has to be there in the last level memory.
Thus in this case,
H1 = 0.9
H2 = 0.6
T1 = 20
T2 = 60
T3 = 12 * 106 ns
So, Tavg = 480,026 ns