Directly the frequency components which constitute 20%, 30% or 40% power can't be computed. Reasonably you can do it.

• First you compute FFT of the signal of interest.
• Then plot the magnitude plot.
• Then manually you can observe whether there are that kind of frequency components or not.
• If there any that kind of components, you can directly list out.
• Coming to the % of energy, you may be required to integrate it over suitable limits to find the % of energy.

yes i am calculating power spectrum.

here i have attached power spectrum.

now i want to calculate frequency which covers 20% area of power spectrum...

thank you Jaya and Jack... i understand it..

Dear Jak i have tried program but giving error for this line

  k=find(D>values(j),1,'first');

 "The input character is not valid in MATLAB statements or expressions."

I think that you need a double loop  across the vector D which is of length of the length(ppx). You need to consider checking each one of the values of the vector D against the criteria of 0.20 0.40 etc.

 Dear jack 

thnk u Jack. i have tried program. now it is working but answer is same for all required frequencies.

variable k [1 1 1 1] having same values so B also have same values [100 100 100 100].

thank you jack for your help