[tex]y\text{ = }13(0.8)^t\text{ (option }D)[/tex]
Explanation:
Exponetial formula:
[tex]\begin{gathered} f(x)=ab^x \\ a\text{ = initial amount} \\ x\text{ = time } \\ b\text{ = growth or decay factor} \end{gathered}[/tex]
a = initial value = 13
rate = r = 20% = 0.2
b = 1 + r
if b > 1 (if it is exponential growth)
if b< 1 (if it is decay)
b = 1 - r
we are told it decays:
b = 1- r
b = 1 - 0.2
b = 0.8
The exponential function becomes:
[tex]\begin{gathered} y=f(t)=a(1-r)^t \\ y=13(1-0.2)^t \\ y\text{ = }13(0.8)^t\text{ (option }D) \end{gathered}[/tex]