Rumus	\({e_{80}} = \sum\limits_{k = 1}^\infty {{}_k{P_{80}}} \)
Kalkulasi	\({e_{80}} = \sum\limits_{k = 1}^\infty {\frac{{{l_{80 + k}}}}{{{l_{80}}}}} \) \({e_{80}} = \frac{1}{{{l_{80}}}}\sum\limits_{k = 1}^\infty {{l_{80 + k}}} \) \({e_{80}} = \frac{1}{{100 – 80}}\sum\limits_{k = 1}^\infty {\left( {100 – 80 – k} \right)} \) \({e_{80}} = \frac{1}{{20}}\sum\limits_{k = 1}^\infty {\left( {20 – k} \right)} \) \({e_{80}} = \frac{1}{{20}}\left( {19 + 18 + 17 + … + 1 + 0} \right)\) \({e_{80}} = \frac{1}{{20}}\left( {\frac{{20(19 + 0)}}{2}} \right)\) \({e_{80}} = 9,5\)
Jawaban	Tidak tersedia pada pilihan