Answer :
Either this is exponential decay at 8% loss per day or it is not. You need to know or decide which it is.
Start with A=p%2Ae%5E%28-kt%29 if this is the kind of model wanted. The variables are
A, amount after passage of time t
p, initial amount when t=0
t, time passage in days
e, base for the Natural Logarithm
k, a constant
First, find the value for k using your 8% loss per day.
For t=1, p=1, A=0.8.
ln%28A%29=ln%28p%29%2Bln%28e%5E%28-kt%29%29
ln%28A%29-ln%28p%29=-kt%2A1
kt=ln%28p%29-ln%28A%29
highlight_green%28k=%281%2Ft%29%28ln%28p%2FA%29%29%29
Substitute the known or given values and evaluate k.
k=%281%2F1%29ln%281%2F0.8%29
k=ln%281.25%29
highlight28k=0.223 29-
The model for your example is A=p%2Ae%5E%28-0.223t%29, or using your given initial quantity of material, highlight%28A=2.01e5E28-0.223t2929
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