Web caching / Networks

Question
We have all learned about web caching that
it reduces the average access time to the Internet. However, we may notice an increase in the access time for those requests that encounter a cache miss, because of the involvement of the cache. Let L is the LAN time, A is the access link time, I is the Internet time, and R is the total response time. Assume that P is the average cache penalty time, then
No cache:
R = L + A + I
Cache and hit:
R = L + P
Cache and miss:
R = L + P + A + I
Assume that L = 10 ms, A = 50 ms, I = 200 ms, and P = 35 ms.
(a) Find the average response time R when no cache is used?
(b) Find the average response time when a cache is used and the hit ratio is 90%?
(c) Find the average response time when a cache is used and the hit ratio is 10%?
(d) Find the hit ratio above which the use of cache is justified.

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