\({U_f} = \left( {\frac{{{q_1}{q_3}}}{{0.8}} + \frac{{{q_2}{q_3}}}{{0.2}}} \right) \times 9 \times {10^9}\) \(= U_f -U_i\) \( = \frac{{{U_f} - {U_i}}}{{{u_i}}} \times 100\)
\( = \frac{{{q_2}{q_3}\left( {\frac{1}{{0.2}} - 1} \right) \times 100}}{{{q_3}\left( {\frac{{{q_1}}}{{0.8}} + \frac{{{q_2}}}{1}} \right)}}\) \(= - 76\% \)