Basic electronics knowledge - revision

9) Proportion Vout/Vin = 470/(120+470)
                                      = 470/590
                                      = 47/59

    Vout = 6 * 47/59
             = 4.77966101695...
             = 4.78 V (3 s.f.)

10) If Vin = 6 V, then
      Vout = 15 * 47/59
               = 11.9491525424...
               = 11.9 V (3 s.f.)
11) Proportion Vout/Vin = 470/(680 + 470)
                                         = 470/1150
                                         = 47/115

      Vout = 6 * 47/115
               = 2.45217391304...
               = 2.45 V (3 s.f.)

12) Vout = 5 V, therefore

       5 = Vin * 47/59
       Vin = 5 * 59/47
              = 6.27659574468...
              = 6.28 V (3 s.f.)