Sum of the last 20 coefficients in the expansion of (1+x)39 , when expanded  in ascending powers of x, is

There are 40 terms in the expansion of (1+x)39 :Sum of last 20 coefficients is S=39C20+39C21+…+39C38+39C39⇒S=39C19+39C18+…+39C1+39C0 Adding the above two expansions we get 2S=39C0+39C1+…+39C39=239⇒S=238.