What does an expression have to have to be a function? what must be true?

This is the number of parts out of 100, the numerator of a fraction where the denominator is 100. submit

Suppose that a households monthly water bill (in dollars) is a linear function of the amount of water the household uses (in hundreds of cubic feet, hcf). when graphed, the function gives a line with slope of 1.65. if the monthly cost for 13 hcf is $46.10 what is the monthly cost for 19 hcf?

Aof 12 , we to a of 4 to on a .of 4 be ?