cache memory

Question

Consider a process where each instruction takes on average 3 cycle and there are 1.8 references to memory per instruction. A program with 50000 instruction is executed on this machine using a split cache of 32KB, obtained a 85% bit rate, 3ns bit time and 21ns miss penalty the execution time for the cache is ___ (μsec)

Comments

CPI = Nominal CPI + Miss penalty For the cache

Cycle time = 3 ns

Miss penalty in cycle = 21 / 3 = 7

cycles CPI = 3 + 1.8 × 0.15 × 7 = 4.89

 Execution time = # of instructions * CPI * Cycle time

For the cache = 50000 × 4.89 × 3ns = 733.5 µs

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“Miss penalty in cycle = 21 / 3 = 7 ”

Can you please explain what happened here?