WE HAVE THOSE TWO MATHEMATIC TYPES HOW CAN WHY COMBINE THEM?

A:B:C=3:2:3 (1)

so    C=A

and 

$\frac{B}{A}=\frac{2}{3}\\ B=\frac{2}{3}*A$

Now

15A+6B+9C=20 (2)

15A+6*(2/3)A+9A=20

15A+4A+9A=20

28A=20

$A=\frac{20}{28}=\frac{5}{7}\\ B=\frac{2}{3}*\frac{5}{7}=\frac{10}{21}\\ C=\frac{5}{7}$