Question 713282
The description or the question is incomplete.  Give information about how the A grade is identified.  So far we can only understand that an A is a score greater than .... what?  You need to know or give the boundary for grade of A.  


The boundary score so far is a variable, let's say A.  A is an known but not given value.  Any score at or above A is a grade of A.  


The next test score earned is X.  Now, is the grading evaluation based on total score points, or is it based on test averages?  We need to know this.


Total test score sum = {{{87+83+89+X}}}.


If grades rely on sum of test scores, then A grade is some quantity of summed test scores, so  for   {{{87+83+89+X>=A}, the value for X must be {{{X>=A-(87+83+89)}}}.


If grades rely on test averages, then the relation is {{{(87+83+89+X)/4>=A}}} for which A is an established value for averages.  {{{X>=4A-(87+83+89)}}} would be the condition needed for earning a score X to make an A grade.