DNA length of one turn is $34\mathrm{ }Å$. It means one turn contains 10 base pairs. So $34\mathrm{ }\mathrm{Å}=10$ base pairs.

Hence, $204\mathrm{ }Å=204\times 10/34=\mathrm{ }60$ base pairs. As given 15% is thymine $=15/100\times \mathrm{ }60=9$ thymine.

Thymine pairs with adenine and hence adenine is also 9. So AT is total of 18. Total $GC=60-18=42$.

Total hydrogen bonds $= 2\mathrm{x}\mathrm{ }\mathrm{A}\mathrm{T}+3\mathrm{x}\mathrm{ }\mathrm{G}\mathrm{C}$ (AT shows double bond and GC shows triple bond) $=\mathrm{ }2\mathrm{ }\mathrm{x}\mathrm{ }18\mathrm{ }+\mathrm{ }3\mathrm{ }\mathrm{x}\mathrm{ }42=36+126=162.$