calculating persistence factor in an RI model

[cfalevel2john]

is the schweser live mock they said the RI will decrease by 20% each year after next. i then used 20% as w. why do we use 1-.2 = .8 as the persistence factor?
question 33

 

[vicky_cool400]

I am having hell lot of problems in this chapter
Is elan video good in this?
Whos is teaching this in elan