A system has a cache, main memory, and a disk. If a referenced word is in cache, 5 nanoseconds are required to access it. The memory access time is 20 nanoseconds, and 1 millisecond is required to access it from the disk if the word is not in main memory. The cache hit ratio is 60% and the main memory hit ratio is 90%. What is the average time required to access a referenced word on this system?