help FM QN, confusing me so bad

[Lauraa10]

Mrs Jones has an english class. In a test her class of 24 students averaged 65%. Another student new to
the class takes the test and the new average is now 64%. The score obtained by the new student was:

A)64%
B)63%
C)60%
D)50%
E)40%

answer is E

[sam.utute]

Bit of logic required:

Let x = sum of all the students scores

x/24 = 65
x = 65*24
x = 1560

So using this information:
Now let y = sum of all the students scores including new student
y/25 = 64
y = 1600

1600-1560 = 40
Therefore the student obtained a score of 40, E.