Answer
Exponential
$y=40*.5^x$
Work Step by Step
The graph is not a line, and the graph appears to not have an axis of symmetry.
Points on the line: $(0,40), (1,20), (2,10)$
As $x$ increases by one unit, $y$ is halved.
$y=ab^x$
$y=40b^x$
$y=40*.5^x$
