$Gs:\begin{cases}m_{dd H_2SO_4 85\%}=100 gam \\m_{dd HNO_3 a\%}=m gam \end{cases}\Rightarrow \begin{cases}\frac{85}{100+m}=\frac{60}{100} \\ \frac{\frac{am}{100}}{100+m}=\frac{20}{100} \end{cases}\Rightarrow \begin{cases}a=68 \\ m=41,667 \end{cases}$$\Rightarrow m_{dd HNO_3}:m_{dd H_2SO_4}=41,667:100$
$\Rightarrow C\%(HNO_3)=68\%$