Cache memory organisation is following hierarchical access as per question description.
So Average Access time can be calculated as
Tavg = H1*T1 + (1-H1) * H2 * ( T2 + T1) + (1-H1) *(1-H2)*H3*(T3+T2+T1)
Where,
H1 = hit ratio in level 1 i.e. cache memory ( 0.9 ) , T1 = Average Access time in level 1 ( 20 ns )
H2 = hit ratio in level 2 i.e. main memory( 0.6 ) , T2 = Average Access time in level 2 (60 ns)
H3 = hit ratio in level 2 i.e. hard disk ( 1 ), T3 = Average Access time in level 3 = (12 ms i.e 12000000 ns )
H3 will be 1 since its last level.
Tavg = 0.9* 20 + 0.1* 0.6*( 60+20 ) + 0.1*0.4*1 * ( 12000000 + 60 + 20 )
= 480026 ns