Alexis bought x boxes of cereal for $3 each, including tax, and paid with a $20 bill. Which expression represents the total amount of change she should receive? 20 – 3x
(3 – 20)x
3x – 20
(20 – 3)x

Scores on a university exam are normally distributed with a mean of 78 and a standard deviation of 8. the professor teaching the class declares that a score of 70 or higher is required for a grade of at least “c.” using the 68-95-99.7 rule, what percentage of students failed to earn a grade of at least “c”?