一條數學問題

By C's saying, we have
C = (A+D)/2..............(1)

By E's saying, we have
E = C+ 2..................(2)

By D's saying, we have
D = (A+B+C+D+E)/5
4D = A+B+C+E.........(3)

Sub (1) and (2) into (3), we have
4D = A+B+(A+D)/2+ (A+D)/2 +2
4D = A+B +(A+D) +2
3D = 2A + B + 2
3D = 2 x 4 + B +2 (because A = 4)
3D = B +10
B = 3D - 10
Since full mark is 10 and mark must be +ve,
D = 4, 5 or 6 (If D = 7, B = 11; wrong!)

Therefore,
If D = 4, B = 2.
If D = 5, B = 5
If D = 6, B = 8

Since B got the highest marks, the answer is that B = 8.

B得8分

A: 4分
B: 8分
C: 5分
D: 6分
E: 7分

9分,(我)得最高分。唔係拿滿分,只是高分!