unit 1 post test year long (total 61 pts)\n15. (3 pts) abby scored 87, 93, 96, and 89 on her first hour math…

Explanation:

Step1: Recall average formula

The average $\bar{x}=\frac{\sum_{i = 1}^{n}x_{i}}{n}$. Here $n = 5$ and $\bar{x}=91$, and the first four scores are $x_1 = 87,x_2=93,x_3 = 96,x_4=89$. Let the fifth - score be $x_5$.

Step2: Set up the equation

We know that $91=\frac{87 + 93+96 + 89+x_5}{5}$. First, calculate the sum of the first four scores: $87+93+96 + 89=365$. So the equation becomes $91=\frac{365+x_5}{5}$.

Step3: Solve the equation for $x_5$

Multiply both sides of the equation by 5: $91\times5=365+x_5$. So $455=365+x_5$. Then subtract 365 from both sides: $x_5=455 - 365=90$.

Answer:

A. 90