As in relative abundance , one is take reference
So,
One is taken as 1:
Other is subtracted from it:
(1 - 0.6011)(atomic mass of Ga-71)
Equation can be written as:
69.723 = (0.6011)(68.9256) + (1-0.6011)x
(1-0.6011) is the percentage abundance of Ga-71 expressed in percentage:
Solving for x
28.2918 = 0.3989 x
x= 70.9246.......