Write through: In write through cache , during writing both memory and cache are updated simultaneously, so write time here is memory access time for a word. On a cache miss, only memory update takes place.
While during reading , a cache block is retrieved from memory and hence , read time on cache miss will be the time to bring a block from memory including cache access time using heirarichal access.
Write Back: Here cache block is replaced only when dirty bit is set, then that block is written back to memory. Write back policy uses write allocate technique.