Answer
$s(x)=1.25x$
Work Step by Step
The cost of riding the subway $x$ times is the product between $x$ and the cost of a token to ride the subway one time:
$$s(x)=1.25x$$
Since the number of rides is only a whole number $0, 1, 2, 3,\dots$, the domain is the set of whole numbers.
$$\text{Domain}=\{0,1,2,3,\dots\}$$
The range consists of multiples of whole numbers:
$$\text{Range}=\{0,1.25,2.50,3.75,\dots\}$$
As the graph consists of separate points, the function is discrete.
