Susan invested some money at 6% annual interest and 800 dollars at 7%. The money earned 128 dollars interest in a year. How much was invested at 6%?

Susan invested some money (M) at 6% annual interest and 800 dollars at 7%. The money earned 128 dollars interest in a year. Then we have:

M x 6/100 + 800 x 7/100 = 128

=> M*0.06 = 128 - 800*0.07 =  128 - 56 = 72

=> M = 72/0.06 = 1200 dollar

Hope this helps!

:)