A double-stranded DNA molecule is 5000 bp long, and 30% of its bases are adenine…

(a) Guanine. Thymine equals adenine: $30\%$. That leaves $100\% - 2 \times 30\% = 40\%$ for guanine and cytosine together, shared equally: $20\%$ guanine.

(b) Cytosine. The molecule has $2 \times 5000 = 10000$ bases. Cytosine, like guanine, is $20\%$ of them: $2000$ bases.

(c) Hydrogen bonds. The $5000$ base pairs are $3000$ A-T pairs (as many as the adenines) and $2000$ G-C pairs. A-T pairs have $2$ hydrogen bonds and G-C pairs have $3$: $3000 \times 2 + 2000 \times 3 = 12000$ hydrogen bonds.

(d) Length. $5000\ \text{bp} \times 0.34\ \text{nm} = 1700\ \text{nm} = 1.70\ \mu\text{m}$.

(e) Turns. $\dfrac{5000\ \text{bp}}{10\ \text{bp}} = 500$ turns. This fits a pitch of $3.4\ \text{nm}$: $500 \times 3.4\ \text{nm} = 1700\ \text{nm}$.