Use Amdahl's Law which states :
Effective speedup = 1 / (1 - f + f / Speedup)
where (1-f) : Fraction of sequential execution
f : Fraction of execution with enhancement(Here enhancement is use of multiple CPUs)
Speedup : Speedup used in parallel part
So according to the question :
80 = 1 / (1 - f + f / 100)
==> f = 395 / 396
==> f = 0.9975
==> (1 - f) [Sequential part] = 1 - 0.9975
==> (1 - f) [Sequential part] = 0.0025
==> Sequential part = 0.25 %
