Tnon-pipeline = sum of all stages delay = 15ns
Tpipeline = max delay among all stages delay = 5ns
Given speedup = 4
Speedup = $\frac{\text{Execution of non pipeline}}{\text{Execution of pipeline}}$
Execution of non pipelined = CPInon-pipelined * Cycle time
Execution of non pipelined = CPInon-pipelined * 15ns
Now,
Execution of pipelined = CPIpipelined * cycle time
In pipelined ideally CPIpipelined = 1
Execution of pipelined = 1 * 5ns
4 = (CPInon-pipelined *15)/(1*5)
CPInon-pipelined = 4/3
CPInon-pipelined = 1.33
Ans = 1.33