Total score obtained = 7+5+10+4+9 = 35
Total points in B and C = 33 +41 = 74
Condition, (35 + 74 +A)/4 >= 90% of 50
or 109+A >= 45*4
or A>= 71
So the student should secure atleast 71 marks to obtain A, which is not possible.
Similarly, the condition to secure B is
(35 + 74 +A)/4 >= 80% of 50
or 109+A >= 40*4
or A>= 51
So in order to obtain grade B, the student should bring atleast 51 marks, which again is not possible.