Final answer:
To find the math class test average after test 2, substitute x=2 into the equation f(x)=0.2x+78, which gives 78.4 as the test average.
Step-by-step explanation:
The question is asking to determine the test average for a math class after completing test 2 using the function f(x)=0.2x+78.
To find the average after the second test, you substitute x = 2 into the equation and do the calculation as follows:
f(2) = 0.2(2) + 78
= 0.4 + 78
= 78.4
Therefore, the test average for the math class after completing test 2 is 78.4, which corresponds to option a) 78.4.