Question 405938
 Ben invested $800, part at 9% per annum and the rest at 12% per annum. After one year the total interest earned was $79.50. How much did he invest?


9% per annum ------  x
12% per annum--------y

x+y =800....1

0.09x+0.12y=$79.50. 
multiply by 100
9x+12y=7950............(2)

multiply (1)--9
-9x-9y=-7200
add to (2)
+3y=750
/3
y = $250 ===> at 12%
Balance $550 ay 9%