واكنش (III) از مجموع واكنش‌های (I) و (II) حاصل می‌شود، پس: $I)\,A+B\to C\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\Delta {{H}_{1}}=60kJ$ $\underline{II)\,C\to D+E\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\Delta {{H}_{2}}=?\,\,\,}$ $III)\,A+B\to D+E\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\Delta {{H}_{3}}=100kJ$ $\Delta {{H}_{1}}+\Delta {{H}_{2}}=\Delta {{H}_{3}}\Rightarrow 60+\Delta {{H}_{2}}=100\Rightarrow \Delta {{H}_{2}}=40kJ$