Why charge on the inner surface has been taken to be -q and on outer surface +q???

Dear Student, The electric field inside a conductor is always zero. So if you are putting a charge q at center of the shell, there should be an equal charge inside to make it zero, so inner surface induce -q charge. Now total charge on Shell is Q so if inner surface has -q outer must have Q+q so that total sum of inner and outer shell will be Q. Hope this help you, for more information you can refer the properties of charges in conductor from our study material. Regards