iridium has two isotopes. with mass numbers 191 and 193, and its average relative atomic mass is 192.23.calculate the relative abundances of the two isotopes

Answer 1

191x + 193(1-x) = 192.23
x = .385

so,
38.5% Ir191
61.5% Ir193

or, since 192.23 is about 5/8 of the way from 191 to 193, 5/8 will be Ir193, or 62.5%. Fairly close ...

Answer 2

ridium has two stable isotopes: 191Ir and 193Ir with masses of 190.9605940 amu and 192.9629264 amu. If the fractional abundance of 191Ir is 0.373 on Earth, complete the expression below that can be used to calculate the average atomic mass.