SOLUTION: Angus invested $18000, part at 3% and part at 5%. If the total interest at the end of the year is $660, how much did he invest at each rate?

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Question 122266: Angus invested $18000, part at 3% and part at 5%. If the total interest at the end of the year is $660, how much did he invest at each rate?
Answer by oscargut(2103) About Me  (Show Source):
You can put this solution on YOUR website!
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