SpeedUp in Computer Organisation

Suppose that a cache is 10 times faster than main memory and suppose that the cache memory can be used 70% of the time. How much speed up do we gain by using the cache?


Let , main memory access time be t units ,
and cache memory access time be t/10 units
Speedup = time without use of cache / time with use of cache
= t / [(0.7)*t/10 +(0.3)*t] = 1/0.37 = 2.7