Answer:
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!
:)
