Calculate Percentage Abundance of each isotope?

I got this question on a chem review:



Chlorine exists as two isotopes, cl-35 and cl-37. the relative atomic mass of chlorine is 35.45. Calculate the percentage abundance of each isotope.



Answer with a full explanation please :)Calculate Percentage Abundance of each isotope?
Let x be the decimal abundance of Cl-35. Then 1-x is the decimal abundance of Cl-37.



35x + 37(1-x) = 35.45

35x + 37 - 37x = 35.45

2x = 1.55

x = 0.775



The percentage abundance of Cl-35 is 77.5%, and of Cl-37 is 22.5%