數學的問題-一堆筆記本

to let a notebook sells $x
therefore
A had $a,he bought $2x(=3a/5)
B had $b,he bought $3x(=4b/7)
C had $c,he bought $2x(=2c/3)
and there are only $550 left
before that,A had
2x=3a/5
a=10x/3
B had
b=21x/4
C had
c=3x
after bought the book,
A had
10x/3 - 2x=4x/3
B had
21x/4 -3x=9x/4
C had
3x-2x=x
sol
550=x+(9x/4)+(4x/3)
550*12=12x+27x+16x
6600=55x
x=120
ans:before A bought notebook,he had
10(120)/3=$400