$$
\begin{aligned}
Q_{1} &=12 \mu \mathrm{F}=12 \times 10^{-6} \mathrm{~F} \\
Q_{2} &=-8 \mu \mathrm{F} \\
&=-8 \times 10^{-6} \mathrm{~F}
\end{aligned}
$$
Initial value of electrostatic force of attraction between both spheres,
$$
\begin{aligned}
F_{1} &=\frac{1}{4 \pi \varepsilon_{0}} \frac{Q_{1} Q_{2}}{r^{2}} \\
&=\frac{1}{4 \pi \varepsilon_{0}} \cdot \frac{12 \times 10^{-6} \times 8 \times 10^{-6}}{r^{2}} \\
F_{1} &=\frac{1}{4 \pi \varepsilon_{0}} \frac{96 \times 10^{-12}}{r^{2}}
\end{aligned}
$$
When both charged spheres are kept in contact, then new charges on the spheres will be
$$
\begin{aligned}
Q_{1}^{\prime}=Q_{2}^{\prime} &=\frac{Q_{1}+Q_{2}}{2} \\
&=\frac{12+(-8)}{2}=2 \mu \mathrm{F}=2 \times 10^{-6} \mathrm{~F}
\end{aligned}
$$
$\therefore$ Final value of electrostatic force between the spheres,
$$
\begin{aligned}
F_{2} &=\frac{1}{4 \pi \varepsilon_{0}} \frac{Q_{1}^{\prime} \times Q_{2}^{\prime}}{r^{2}} \\
&=\frac{1}{4 \pi \varepsilon_{0}} \cdot \frac{2 \times 10^{-6} \times 2 \times 10^{-6}}{r^{2}} \\
F_{2} &=\frac{1}{4 \pi \varepsilon_{0}} \cdot \frac{4 \times 10^{-12}}{r^{2}}
\end{aligned}
$$
From Eqs. (i) and (ii), we get
$$
\begin{array}{ll}
\therefore \quad & \frac{F_{1}}{F_{2}}=\frac{\frac{1}{4 \pi \varepsilon_{0}}}{} \cdot \frac{96 \times 10^{-12}}{\frac{1}{4 \pi \varepsilon_{0}}} \cdot \frac{4 \times 10^{-12}}{r^{2}} \\
\Rightarrow \quad & \frac{F_{1}}{F_{2}}=\frac{24}{1} \\
\Rightarrow \quad & F_{1}: F_{2}=24: 1
\end{array}
$$