How efficient is this assembly line for the table below?

Efficiency $(\%)=\frac{({\displaystyle \underset{?}{\overset{?}{}}}t)}{NC}$

$\backslash$table$[[$Task$,\backslash$table$[[$Time$],[($Minute$)]],$Predecessors$],[$A$,4,$None$],[$B$,5,$A$],[$C$,6,$A$],[$D$,7,$B$,$ C$],[$E$,3,$D$]]$

$66\mathrm{.}7\%$

$71\%$

$83\%$

$100\%$