# How do you calculate when effective access time is greater than cache access time?

I'm having trouble understanding how to calculate effective access time, hit ratios and cache access times. I'm unfamiliar with the concepts and would love help, or an explanation on how to solve this problem:

Consider a memory system with a main memory of 16 MB and a cache memory of 8,192 bytes. The cache cost is 0.02 cents/bit while the memory cost is 0.001 cents /bit. The cache access time is 50 nanoseconds whereas the memory access time is 400 nanoseconds. If the cache hit ratio is 90% what percent is the effective access time greater than the cache access time? What is the average cost per byte of memory for the system?

Any help is truly appreciated.

• Effective access time means expected memory access time. – randomsurfer_123 Sep 2 '15 at 21:53
• Why don't you edit your post to ask one question about one concept that you don't understand, and edit out the rest? Pick a concept, tell us what self-study and research you've done, and ask a specific question about what specifically confuses you. That might prove helpful to others in the future. In contrast, giving us a copy-paste of your exercise is not a good fit for this site. We want to help you understand concepts, but solving your exercise for you is not helping you or anyone else. – D.W. Sep 3 '15 at 0:46