Let we have X of Tl-205 so the percent of Tl-203 will be 100-X
Average mass of Thallium is 204.4
Mass of Tl-205=[tex]\frac{X\times 205}{100}[/tex]
Mass of Tl-203=[tex]\frac{100-X\times 203}{100}[/tex]
So,
[tex]\frac{100-X\times 203}{100}+\frac{X\times 205}{100}=204.4[/tex]
205X +20300-203X=20440
X=70
So the percent or abundance of Tl-205 is 70 while that of Tl-203 will be 30.
