Question 641182
To solve, we first need to know how to find the average of a group of numbers.  To find the average of a group of numbers, you add all the numbers together and divide that answer by the number of numbers you have.  For instance, to find the average of 1, 2, 3, 4 and 5, we would add all the numbers together, which would equal 15.  We would then divide that number by 5, because there are 5 numbers total:  15/5 = 3.  So the average is 3.  

Now, to find the average in your problem, we need to look at what we know:

We know there are a total of 5 tests
We know 4 of the scores
The average of all 5 tests is 88

In other words:

{{{(86+90+84+91+x)/5 = 88}}}

If we multiply both sides of this equation by 5, we will get rid of the
fraction, and leave us with:

{{{86+90+84+91+x = 88*5}}} =

{{{351+x=440}}}

Finally, if we subtract 351 from both sides, we will be left with what x equals, which will be our 5th test score:

{{{351-351+x=440-351}}} =
{{{x = 89}}}

FINAL ANSWER:  Bill must earn an 89 on his fifth test