Suppose the mass spectrum of a hypothetical monatomic element x contains a signal at mass number 13 and another of identical height at mass number 15.
how many isotopes are present and why
what's the fractional abundance of the isotopes. why?

For this case, Only 1 isotope would be present, i.e. the principal element with mass M=13 and then one isotope at mass M+2. We are assuming that the principal element is the one that is the lowest mass - by definition, an isotope is one where there are additional neutrons - hence the mass increases, but the proton count is the same.