To answe this question, we will assume that the percentage abundance of 14n is y. Since the percentage abundance of 14n + percentage abundance of 15n = 100% = 1
Therefore, percentage abundance of 15n = 1 - y.
Now we know that the average atomic mass of nitrogen is 14.0067
Therefore:
average atomic mass of nitrogen = atomic mass of 14n x its percentage abundance + atomic mass of 14n x its percentage abundance
14.0067 = 14.003074002 y + 15.00010897 (1 - y)
14.0067 = 14.003074002 y + 15.00010897 - 15.00010897 y
14.0067 - 15.00010897 = 14.003074002 y - 15.00010897 y
0.99703497 y = 0.99340897
Therefore y = 0.99636
Based on this, the percentage abundance of 14n is 0.99636x100= 99.636%
while percentage abundance of 15n = 100% - 99.636% = 0.364%