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Evaluate the expression for the given value of the variable 7a - 4a for a =8

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?

What is y times x by? y is __ times x