The energy required to knock out the electron in the third orbit of a hydrogen atom is equal to

$13.6 eV$ $+\frac{13.6}{9} eV$ $-\frac{13.6}{3} eV$ $-\frac{3}{13.6} eV$

$+\frac{13.6}{9} eV$

Energy required to knock out the electron in the nth orbit $=+\frac{13.6}{n^2} eV \Rightarrow$ $E_3=+\frac{13.6}{9} eV$