Question 884046
We can do this algebraically as well as using trial and error.  Trial & error is ok here because there are relatively few numbers to test, but for most of you it will be quicker using algebra.
 
So here's what you can do.  The unknown {{{x}}} is the final exam percentage score, which could range from 0% to 100%.  Immediately we might note mentally to ourselves that it could be the case that we might need {{{x>100}}} to achieve an overall 75%.  But this fear turns out to be unfounded, since already your average is {{{ (68+78+81)/(3) = 227/3 = 75}}}{{{ (2)/(3) }}}.  So to maintain an average above 75 you will <i>not</i> need a final exam score over 75.
 
So we will have a final overall grade from the average of the four exams, which we must equate to 75, so
{{{ 75 = (68+78+81+x)/(4) }}}
thus {{{ 4 * 75 = 68+78+81+x }}}
thus {{{ 300 = 68+78+81+x }}}
thus {{{ x = 300 - (68+78+81)}}}
thus {{{ x = 300 - 227 }}}
thus {{{ x = 73. }}}