chemistry
posted by Luc .
An element with relative atomic mass 20.8 is found to contain 2 isotopes having a relative atomic mass of 20 and 21.Find the % composition of the element.

Relative atomic mass = 20.8 g
Let the fraction that is element 20 = x
thus the fraction that is element 21=1x
so 20x +21(1x) = 20.8
20x + 21 21x=20.8
1x=0.2
x=0.2 and 1x=0.8
so 20% element 20 and 80% element 21. 
20.8%

44%