Nitrogen has two naturally occurring isotopes, n-14 and n-15. the atomic mass of nitrogen is 14.007 amu. which isotope is more abundant in nature? explain

Suppose that:
Mass of N-14= 14 
and Mass of N-15= 15 

fraction N-14 x 14 + fraction N-15 x 15 = 14.007 

now.. if the fractions were =, that is if fraction N-14 = fraction N-15 = 0.5, then... 
0.5 x 14 + 0.5 x 15 = 14.5 

more N-15 would make that number larger.. 
0.3 x 14 + 0.7 x 15 = 14.7
 let x = abundance of N-14, then (1-x) = abundance of N-15, and we could solve for x. 
(X)x14 + (1-X)x 15 = 14.007

14X + 15 - 15X = 14.007 
X = 0.993 

the relative abundance of N-14 = 99.3% 
the relative abundance of N-15 = 0.7%
So, according to above explanation N-14 is abundant