ANSWER
[tex]f(x)=80( \frac{1}{4}) ^{x-1}[/tex]
EXPLANATION
The term of the sequence are;
[tex]80,20,5,...[/tex]
The first term of the sequence is,
[tex]a_1=80[/tex]
The common ratio is
[tex]r = \frac{20}{80} [/tex]
[tex]r = \frac{1}{4} [/tex]
The sequence is given by the formula
[tex]f(x)=a_1r^{x-1}[/tex]
When we substitute the values into the formula we get,
[tex]f(x)=80( \frac{1}{4}) ^{x-1}[/tex]
The correct answer is A
