Answer:
Let x be the percentage score he got in fifth test,
According to the question,
His score in first, second, third and fourth test are 80%, 84%, 76% and 77% respectively,
Thus, the average of his total score in the five test,
= ![\frac{80+84+76+77+x}{5}](https://tex.z-dn.net/?f=%5Cfrac%7B80%2B84%2B76%2B77%2Bx%7D%7B5%7D)
But, Again according to the question,
The average of the five grades as greater than or equal to 80%
That is,
![\frac{80+84+76+77+x}{5}\geq 80](https://tex.z-dn.net/?f=%5Cfrac%7B80%2B84%2B76%2B77%2Bx%7D%7B5%7D%5Cgeq%2080)
⇒ ![\frac{317+x}{5}\geq 80](https://tex.z-dn.net/?f=%5Cfrac%7B317%2Bx%7D%7B5%7D%5Cgeq%2080)
⇒ ![317 + x \geq 400](https://tex.z-dn.net/?f=317%20%2B%20x%20%5Cgeq%20400)
⇒ ![x\geq 83](https://tex.z-dn.net/?f=x%5Cgeq%2083)
Thus, the least score he can get in the fifth test is 83.