The given situation is shown in the figure below,



If the system moves upward with acceleration $a \mathrm{~m} / \mathrm{s}^2$ on the application of $104 \mathrm{~N}$, then the equation of motion for $8 \mathrm{~kg}$ block is
$$
\begin{aligned}
& 104-T_1-8 g \sin 30^{\circ}=8 a \\
& \Rightarrow \quad 104-T_1-8 \times 10 \times \frac{1}{2}=8 a \quad\left[\because g=10 \mathrm{~m} / \mathrm{s}^2\right] \\
& \quad 64-T_1=8 a
\end{aligned}
$$
Equation of motion for $5 \mathrm{~kg}$ block is
$$
\begin{array}{r}
T_1-T_2-5 g \sin 30^{\circ}=5 a \\
T_1-T_2-25=5 a
\end{array}
$$
Equation of motion for $3 \mathrm{~kg}$ block is
$$
\begin{aligned}
T_2-3 g \sin 30^{\circ} & =3 a \\
T_2-15 & =3 a
\end{aligned}
$$
Adding Eqs. (i), (ii) and (iii), we get
$$
24=16 a \Rightarrow a=1.5 \mathrm{~m} / \mathrm{s}^2
$$
Hence, acceleration $a$ of block is $1.5 \mathrm{~m} / \mathrm{s}^2$.