The abundance for the isotopes = 65%(isotope 1) and 35%(isotope 2)
The elements in nature have several types of isotopes
Isotopes are atoms whose no-atom has the same number of protons while still having a different number of neutrons.
So Isotopes are elements that have the same Atomic Number (Proton)
Atomic mass is the average atomic mass of all its isotopes
Mass atom X = mass isotope 1 . % + mass isotope 2.%
The average atomic mass of fictitious element Z is 261.4 amu
isotopes 1 = 260, x%
isotopes 2 = 264, 100-x%
[tex]\tt avg~atomic~mass=260x\%+264(100-x)\%\\\\261.4=2.6x+264-2.64x\\\\0.04x=2.6\\\\x=65[/tex]
