execution time

Question

how to solve this?

Answer 1

M/M Ref - 4 cycles
ALU Opn - 2 cycles

Some Basic Understanding ( for 1 word ) :

[[]] - Two Memory References  - 8 CPU Cycle  

[] - One Memory References   - 4 CPU Cycle

[] with ALU Operation : Memory Access with ALU Operation : 4+2 CPU Cycle

Cycle Time = 1/frequency
           =1/2 = 0.5 ns

I1: 4x8 = 32
I2: 2x4 = 8
I3: 1x(4+2) = 6
I4: 1x(4+2) = 6
I5: 4x(4+4) = 32
I6: 0

total = 84*.5ns = 42ns

My Answer is not matching but explanations says like this. Please help if I am wrong anywhere.