problem solving spectral roll off equation in matlab

can any one help me out with this 
try to do a music/speech discriminator 
but got problem to make a spectral rolloff feature
dont know how to solve the equation of spectral roll off

can some one help here please

set M[f]=3,2,5,6,10,4,7,9,1,13

sum(M(1:n)=0.85*(sum(M[f]))

need to find "n"