Question

DUPLICATE
Gallium has two naturally occurring isotopes (Ga-69 and Ga-71) and has an atomic mass of 69.723 amu. The mass of Ga-71 is 70.925 amu, and its natural abundance is 39.892%.
Calculate the atomic mass (in amu) and natural abundance (in %) of the second isotope.

Answer 1

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.......