75% by mass means : ∵75 gHNO3 in 100 g solution. ∴30 gHNO3 in 75100×30 g solution. $\begin{aligned}
& \mathrm{m}{\text {solution }}=\frac{100 \times 30}{75} \mathrm{g} \
\because \quad & \mathrm{V}{\text {sol }}=\frac{\mathrm{m}{\text {sol }}}{\mathrm{d}{\text {sol }}}=\frac{100 \times 30}{75 \times 1.25}=32 \mathrm{~mL}
\end{aligned}$