Finite Math: score needed on 5th test for overall ave of 90

[ivysmimi]

I hope I am under the right "department" for this question. If not, Ted & viewers---I'm sorry. Please bear with me.

I need to know how to work the following problem:
After 3 tests a student has an average of 92. He made 77 on his 4th test. What score would he need on the 5th test to have an average of 90?

If each of the 3 test grades had been given I would have been able to work this problem. Unfortunately for me, they were not. I have no clue how to even begin. Any help would be greatly appreciated. THANKS!!!!!!!!!!!!!!

 

[soroban]

Hello, ivysmimi!

After 3 tests a student has an average of 92. He made 77 on his 4th test.
What score would he need on the 5th test to have an average of 90?

We don't need the first three tests scores . . .

\(\displaystyle \text{If his average is 92, they got it from: }\;\frac{\text{sum of the first three}}{3} \:=\:92\)

\(\displaystyle \text{Hence: }\:\text{sum of the first three} \:=\:3(92) \:=\:276\)


\(\displaystyle \text{His final average is: }\;\frac{\text{sum of the first three} + \text{4th} + \text{5th}} {5}\)

\(\displaystyle \text{Our equation is: }\;\frac{276 + 77 + x}{5} \;=\;90\)

Got it?