Non Pipelined:
We do not consider pipeline registers delays as there is no pipeline. So,
Time taken by 1 instruction to execute = (20 + 50 + 60 + 50 + 20) = 200 ns
Pipelined:
Here we will consider pipeline registers delay.
So considering delays, time taken by stages IF, ID, EX, MEM, WB are 30 ns, 70 ns, 70 ns, 70 ns, 30 ns respectively.
In pipelining we consider CPI = 1.
So time taken by 1 instruction to execute = max( IF, ID, EX, MEM, WB) = 70 ns.
Speed up = Time taken by non pipelined instruction / Time taken by pipelined instruction = 200/70 = 2.857