Yes, this is a function because the mapping is one to one. The domain is the the different inputs that can be put into the function, so the domain is {1,2,3,4,5}. The range is the different outputs the function can produce. So the range would be {40,80,120,160,200}.
Whenever the same input can produce 2 or more different outputs (i.e. not unique), it is not a function. If every input produces a single output, then it is a function. Note that in a function different inputs could produce the same value of output and remains a function, as long as EVERY input produces a SINGLE output.
Example: for the relation f(x) {(1,1),(2,2).(3,1).(4,0)} Each input produces a single output, so f(x) is a function. for the relation g(x)={(1,1),(1,2).(2,1),(2,3)} We see input of 1 has an output of either 1 or 2, so it is no longer a SINGLE output, so g(x) is not a function.
In Marco's case, each input produces a single output. So it is a function.
In Shelia's case, it is almost the same, except that an input of 3 could produce 3 or 6 as output, so it produces more than one output for the same input. So Shelia's case is not a function.
