Question 122266
Let C1 and C2 the amounts of the parts then

C1 + C2 =18000   and  C1(0,03)+C2(0,05)=660  then C2=(18000-C1) then
 C1(0,03)+(18000-C1)(0,05)=660 then  -0,02C1 +900 =660
then  -0,02C1=-240 

then C1=12000 and C2=6000