chemistry
posted by Riana .
Element x is composed of isotopes x56 mass 56.0 u and x59 mass 59.0 u. What is the percentage abundance of each of these isotopes if the atomic mass is 57.3 u?

Let f = fraction of x56,
then 1f = fraction of x=59

56f + 59(1f) = 57.3
solve for f and 1f and convert each to percent.
Note: (we could have said f = % of 56 and 100f  % of 59 but then we would have to multiply 57.3 by 100 and solve for f and 100f; the answer would have been directly in percent.)