Answer:
The atomic mass for element Z is 19.9
Explanation:
Mass number and relative abundance of the first isotope is 19 and 55% respectively.
Mass number and relative abundance of the second isotope is 21 and 45% respectively.
Mass number of element Z = (0.55Ă—19) + (0.45Ă—21) = 10.45 + 9.45 = 19.90