Therefore, Stance = Double Support + Swing

But Swing = 100 - Stance

Therefore, Stance = Double Support + (100 - Stance)

or, Stance = (Double Support + 100)/2

                            = (30 + 100)/2 = 65%

Or you could do it by:

Double Support = 2Stance - 100

i.e.                     Stance = (100 + Double Support)/2
                                    = 130/2 = 65%