Which relations are functions?

Monthly water bills for a city have a mean of $108.43 and a standard deviation of $32.09. find the probability that a randomly selected bill will have an amount greater than $155, which the city believes might indicate that someone is wasting water. would a bill that size be considered unusual?

:24% of students in a class are girls. if there are less than 30 students in this class, how many students are there? how many girls are there?

What is the slope of the line (-7,2) (4,6)