From the definition of a function, a function associates every element of the domain to a unique element in the codomain.
In other words, if two elements are equal in the domain, then their images should also be equal.
Since, x represents the number of cars, we consider the universal set as N, the set of all natural numbers.
Let x, y ∈ N and x = y
Then, 0.75x = 0.75y
⇒ f(x) = f(y)
Therefore, images are equal, whenever elements are equal.
Hence, the function f(x) = 0.75x is a function. Also, note that the domain of the function is the set of all natural numbers and the range is the set of real numbers.
