Answer :
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.