Question

johns math course requires 5 tests. his first 4 grades were 69, 84, 88, and 80.
to end up with an average score of at least 79.2, what is the lowest score john will need on the fifth test?

Explanation:

Step1: Calculate the total score needed

The formula for the average is $\text{Average}=\frac{\text{Sum of scores}}{\text{Number of scores}}$.
If the average of 5 tests is at least $79.2$, then the total score needed is $79.2\times5$.
$$79.2\times5 = 396$$

Step2: Calculate the sum of the first 4 scores

Sum of first 4 scores: $69 + 84+88 + 80$.
$$69+84 + 88+80=(69 + 84)+(88 + 80)=153+168 = 321$$

Step3: Find the score on the fifth test

Let the score on the fifth test be $x$.
We know that $321+x\geq396$.
Subtract 321 from both sides: $x\geq396 - 321$.
$$x=396-321=75$$

Answer:

$75$