Assume that a system is using write-through cache. It has 70% write hits. (Further assume we are using no write allocate policy)
Average access time in case of write = $0.30\left (0.70 * (100) + 0.30 * (100) \right ) = 30$ ns
The cache operates in look-aside mode with a read-hit ratio of 80%
Average access time in case of read = $0.70\left (0.80 * (20) + 0.20 * (100) \right ) = 25.2$ ns
Average access time of the system is = $30 + 25.2 = 55.2$ ns.