The group of points {(0, 1), (0, 5), (2, 6), (3, 3)} is not a function, but the group of points {(1, 4), (2, 7), (3, 1), (5, 7)} is a function. what do you notice about the two groups of points? what do you think it means to be a function?

An x value can only correspond with one value of y in order to be a a function. Although, a y value can correspond with multiple x values. The x=0 in the first set of points corresponds with two different y values: y=1 and y=5. In the second set, none of the x values correspond with multiple y values. If the sets are graphed, the first set of points would pass the vertical line test. If the vertical drawn intersects the graphed segment multiple times, the graph is not a function. 

25 is obviously the correct answer