While Chargaff's rules applies to dsDNA and can be used to calculate proportion of other nucleotides, the same can’t be applied here.
The simple reason is that when you consider a single strand of DNA, you cannot predict the proportion of other nucleotides based on one.
For example, let’s take a DNA molecule with 100 nucleotides. On one strand you have 20 cytosine. From this, you can only tell that there will be 20 guanines on the complementary strand (complementary to cytosines that we know about). But looking at rest of the 80 nucleotides on our strand, it can be any of the three other nucleotide.
This is a trick question, which can be answered through logical reasoning only.
Hope it helps.
