Question 104106
Let the 5th score be x.
Now the average(mean)= (total of all values)/(the number of values).

Now the total of all values in your question is 80+96+78+87+x and the number of values in your question is 5. You are also told that the average value has to be 84.

So the average 84=(80+96+78+87+x)/5.
This means that 84=(341+x)/5.

Now multiply both sides of this equation by 5 and you get:
                          84*5=341+x
Which leads to             420=341+x
Now take away 341 from both sides of the equation and you get:
                           420-341=x

And so x=420-341=79.

The 5th test score then is 79ANS