Question 292648
(83+76+69+x)/4>=81
(228+x)/4>=81
228+x>=4*81
228+x>=324
x>=324-228
x>=96 ans. for the minimum grade on the fourth test to maintain the 81 ave.
Proof:
Let x=96
(83+76+69+96)/4>=81
324/4=81
81=81