sorry,im new user in this forum, so,i don't know how i must reply
thank you for all explanations. really, I'm feeling that there are some matters got more clear.
regarding those lecturers, they were believing that 90% hit ratio is divided between read and write , i.e. it didn't be hit ratio for only read.
by that, 72 will be hit for read and 18 for write, but this absolutely isn't convincing,because, it is mentioned that hit just for read.

anyway, I'm too grateful for you.
but, there is just another thread relate "how calculate the size of cache memory".
i hope , you can provide me with some information about the size of cache which is be a different, depending on type of cache.
keep in your mind, I didn't leave a web is relating cache ,I didn't browse it to look for a way to calculate the size.but, unfortunately there is no such topic.
I have searched since long time but there isn't any information about that