
$\therefore$ $\Delta {Q_{AB}} = \Delta {U_{AB}} = 50J$ (Given)
${U_A} = 1500J$ $\therefore$ ${U_B} = (1500 + 50)J = 1550J$
$\Delta {W_{BC}} = - \Delta {U_{BC}} = - 40J$(Given)
$\therefore$ $\Delta {U_{BC}} = 40J$ $\therefore$ ${U_C} = (1550 + 40)J = 1590J$


