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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

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