Answer
$f(x)=5\left(0.25^{x}\right)$
Work Step by Step
Exponential models have form: $f(x)=y=Ab^{x}$
When $x$ increases by one unit, the value of $f$ is multiplied by $b.$
$A=f(0)$
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Decreasing f by factor 0.75 means multiplying f with 0.25.
(taking away three quarters, one quarter remains.)
So, $b=0.25.$
$A=f(0)=5$ (given) so
$f(x)=Ab^{x}=5\left(0.25^{x}\right)$
