By default Hierarchial access
Tavg = cache miss + main memory hit
Cache miss = cache hit ratio * cache access time = 0.8 * 30ns = 24ns
Main memory hit = (1-cache hit ratio) * (main memory hit ratio) * (main mem access time + cache mem access time)
= (1-0.8) * (1) * (150ns+ 30ns)
= 36ns
(Main memory is final level of hierarchial access so main mem hit ratio = 1)
Therefore,
Tavg = 24+36 = 60ns