A special $4-$year endowment insurance is issued on $(70).$ The death benefit schedule is as follows:

$\backslash$table$[[$year$,$benefit$],[1,50],[2,60],[3,70],[4+,80]]$

You are given:

$($i$)$ Benefits are paid at the end of year.

$($ii$)$ Mortality follows de Moivre's law with $=120.$

$($iii$)i=0.$

Calculate the variance of this insurance.