Question 530658
Let the score on the fourth test be x, then...
{{{(83+78+77+x)/4 = 80}}} Simplify.
{{{(238+x)/4 = 80}}} Multiply both sides by 4.
{{{238+x = 320}}} Subtract 238 from both sides.
{{{x = 82}}}
The student must earn a score of 82 on the fourth test to achieve an overall average of 80.