Add grades for  qz1,qz2,midterm and call  sum G.
That is G= qz1+qz2+midterm  [ G is at most 500]

(if you missed one of the quizzes extrapolate based on
the non-missed quiz. Indicator below is estimate on
performance SO FAR in the course based only on three
pieces of information; 3 more quizzes and 2 mini-projects
plus the final are to be given.)

 500 >= G >  425  : A
 425 >= G >  350  : B+ or B
 350 >= G >  250  : C+ or C
 250 >= G >  0    : D  or F