we know that
Each year, its value is 80% of its value the year before
that is the same as
Each year the value decreases by 20%
we have an exponential decay function of the form
where
y is the value of the computer laptop
x is the number of years
r is the rate
a is the initial value
so
we have
a=$3,200
r=20%=20/100=0.20
substitute
For y=$700
substitute in the equation above
Apply log on both sides
therefore
The answer is 7 years