Question 498700
First, let's find the sum of the first three test grades, which we'll call a, b, and c.
{{{(a+b+c)/3 = 78}}} this is the average of the first three grades. Multiply both sides by 3.
{{{a+b+c = 234}}} Now let's call the fourth test grade x, so...
{{{(a+b+c+x)/4 = 80}}} This is the desired average of the four test grades. But {{{a+b+c = 234}}} so substitute:
{{{(234+x)/4 = 80}}} Multiply both sides by 4.
{{{234+x = 320}}} Subtract 234 from both sides.
{{{highlight(x = 86)}}}
You will need to make a grade of 86 on the fourth test to obtain a final average of 80.