Anew synthetic element has two isotopes. if the first isotope has a mass of 259.98 amu and a percent abundance of 62%, what is the percent abundance of the second isotope?

The sum of the percentage abundance of these two isotopes of the new synthetic element should be equal to 100. If we let x be the percent abundance of the second isotope, we have the equation,
                                           62 + x = 100
The value of x from the equation is 38. 

The molecules contain like 54 of water