Respuesta :

Answer:

204.4 amu.  

Explanation:

30% of thallium will be multiplied by the mass number of thallium-203(203). 70% thallium will be multiplied by the mass number of thallium-205(205).

Then you will add them up.

So, (0.30x203) + (0.70x205) = 204.4 amu.