Heat of combustion of A(s) $-$10kcal ${\mathrm{mol}}^{-1}$ and that of B(s) is $-$15Kcal ${\mathrm{mol}}^{-1}.$ Mixture containing 3 moles A(s) and x moles B(s) on combustion released 105 kcal of heat. What is the value of x?