Timmy has three times as much money invested at 5% bonds as he has in stocks paying 7%. if his annual income from the stocks and bonds is $158.40, how much has he invested in bonds?

Let b repesent the amount Timmy has invested in bonds. Then b/3 represents the amount Timmy has invested in stocks. His earnings are
  158.40 = 0.05*b + 0.07*(b/3)
  158.40 = (0.22b)/3 . . . . . . . . . . combine terms
  158.40*3/0.22 = b = 2160

Timmy has $2160.00 invested in bonds.