Question 514009
Let x be the grade on the last test.  The average of the five test grades is therefore:

{{{ (88 + 85 + 91 + 92 + x)/5 }}}

We want this average to be 90, so we set it equal to 90 and solve for x:

{{{ (88 + 85 + 91 + 92 + x)/5 = 90 }}}
{{{ (356 + x)/5 = 90 }}} (combine like terms)
{{{ 356 + x = 450 }}} (multiply both sides by 5)
{{{ x = 94 }}} (subtract 356 from both sides)

So you need a 94 on the last exam to get a 90 average on all five exams.  Make sure to check that the average of the five exams is indeed 90!