Hi Caroline,

This is definitely a challenging question. The overall idea is that because the 2 rounds of division is occurring in medium containing 14N, a nitrogen of smaller molecular weight, the mass is lower.

Lets take one cell from the first medium with 15N. During the synthesis phase of the first cell division, the 15N chromosomal strands will duplicate each have a 14N strand creating a mixed situation. So in one chromosome, one sister chromatid would be 15N and the other would be 14N. So since half of the genome are 15N and the other half 14N, we can do (5.5 + 5.4)/2 = 5.45.

Now during the synthesis phase of the 2nd cell division, there are 15N chromosomes and 14N chromosomes, so when they replicate we'll have 15N-14N sister chromatids of chromosomes and 14N-14N sister chromatids of chromosomes. This would mean half the genome will be the mixed situation as above of 5.45 and the other half exclusively 14N genome, with a mass of 5.4 fg.

Its definitely easier to draw it out than read an answer. So I'd definitely try doing that to add to your understanding!

Hope this helps!