[peroxide62]

IX2F
Iodine has a stable naturally-occurring isotope (127I atomic mass 126.904 amu).
Fluorine has a stable naturally-occurring isotope (19F atomic mass 18.988 amu).
Element "X" has two stable naturally-occurring isotopes

What is the atomic mass of the less massive isotope of element X?
What is the percent natural abundance of the less massive isotope of element X?

Can someone show me how to do these two questions?
I don't know where to begin.

Thanks

[Enthalpy]

Welcome, peroxide62!

F and I being mono-isotopic, the whole mass fluctuation results from the element X.
Then, you have 2 atoms of X per molecule, hence the 3 molecular masses from 2 different element masses.
The rest is additions and subtractions.