Heavy and Light DNA strand calculation... help!

[Mallen]

Here is the question...
After many generations of growth in medium containing 14N, the medium was replaced with new medium containing 15N.
After 5 generations, what is the ratio of light:heavy DNA strands...?

I answered...
Initial ratio (Light : Heavy) 0:2
End Round 1 2:2
End Round 2 2:6
End Round 3 2:14
End Round 4 2:30
End Round 5 2:54 or 1:27

But this is wrong and I don't know why.....!!!!
Please help.
Thanks.