Why is there not an associative property of subtraction?

Given this information, which expression must represent the value of b?

Acircle has a radius of 5/6 units and is centered at (3.6, 7.8) write the equation of this circle

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?