For a set of pairs of xy values to be a function, there must be one value of y for each value of x (the domain)
For the set of values
{(0,0), (1.5, 10), (1.5, 20), (3, 30)}
The input x=1.5 is repeated, this means that for this value corresponds two outputs: y=10 and y=20.
This is NOT a function, because two ordered pairs have an x-coordinate of 1.5