Which of these equations represent functions where x is the input and y is the output? select each correct answer. a- x=5 b- y=4 c- y=3x d- x=3y e- x+y=5

The first equation is a constant.  There is no input necessary, and the output remains 5.
The second equation is also a constant.  The output is 4.
In the third equation, y depends on what value of x you use.  x is the input and y is the output.
In the fourth equation, x depends on what value of y you use.  The way it is written, y is the input and x is the output.
In the last equation, either x or y could be the input.  The other variable would change depending on which variable you replace with a number.