Thermodynamics

The bond dissociation energies of $A_2, B_2$ and $A B$ are in the ratio $1: 4: 2$. If $\Delta H$ for formation of $A B$ from $A_2$ and $B_2$ is $-100 \mathrm{~kJ} / \mathrm{mol}$, calculate the bond dissociation energy of $B_2$.