What is the total work done in the given $P-V$ graph?

$A$ thermodynamic process is shown in the figure. The pressures and volumes corresponding to some points in the figure are:
$P_A = 3 \times 10^4 \, Pa, \, P_B = 8 \times 10^4 \, Pa$ and $V_A = 2 \times 10^{-3} \, m^3, \, V_D = 5 \times 10^{-3} \, m^3$
In process $AB$,$600 \, J$ of heat is added to the system and in process $BC$,$200 \, J$ of heat is added to the system. The change in internal energy of the system in process $AC$ would be ...... $J$.

$A$ thermodynamic system is taken through the closed cycle $PQRSP$ process as shown in the figure. The net work done by the system is ...... $J$.

$A$ thermodynamic system is taken from an original state to an intermediate state by the linear process shown in the figure. Its volume is then reduced to the original value from $E$ to $F$ by an isobaric process. Calculate the total work done (in $J$) by the gas from $D$ to $E$ to $F$.

$n$ moles of a van der Waals' gas obeying the equation of state $\left(p+\frac{n^2 a}{V^2}\right)(V-n b)=n R T$,where $a$ and $b$ are gas-dependent constants,is made to undergo a cyclic process that is depicted by a rectangle in the $p-V$ diagram as shown below. What is the heat absorbed by the gas in one cycle?

In the following figures $(a)$ to $(d)$,the variation of volume with change of pressure is shown. The gas is taken along the path $A B C D A$. The change in internal energy of the gas will be .......