A hypothetical reaction A 2 B proceeds through the following...

One mole of $A$ as reactant So, we write down equation (i) Two moles of $B$ as product, so we multiply equation (iii) by (b). Finally, we add the three equation term-by-term $$\begin{array}{ll}A \longrightarrow C & q_1 \\ C \longrightarrow D & q_2 \\ D \longrightarrow 2 B & 2 q_3 \\ A \longrightarrow 2 B & q_1+q_2+2 q_3\end{array}$$ Therefore, the heat of hypothetical reaction is $q_1+q_2+2 q_3$.