Question

A student has gotten the following grades on his tests. What is the minimum grade he must get on the last test in order to have an average of 84​?

A student has gotten the following grades on his tests. What is the minimum grade he must get on the last test in order to have an average of 84​?
78​, 90​, 79​, 88.

Answer 1

85

Explanation:

`(78+90+79+88+x)/5=84`

`(78+90+79+88)/5+x/5=84`

`67+x/5=84`

`x/5=17`

`x=85`

Answer 2

`85`

Explanation:

For now, let's call the unknown last test grade `x`. Thus, we have

`(78+90+79+88+x)/5=84`

This equation makes sense because we are essentially finding the average of the test scores. Let's simplify this further to get

`(335+x)/5=84`

Multiplying both sides by `5`, we get

`335+x=420`

Lastly, subtracting `335` from both sides, we get

`x=85`

This is what the final test grade has to be. Let's find the averages now to ensure that this number makes sense:

`(78+90+79+88+85)/5=420/5=color(blue)(84)`

Our average is `84` with a final test grade of `85`.

Hope this helps!