gauss law can be easily apllied ...its impossible for human to solve this ques otherwise...for charges on edges and surface put four cubes and two cubes resp...just like we did it in solid state...charges on edges are shared by four equivalent cubes and on faces by two equivalent cubes...so gauss law and those structure in solid state are somewhat related...and similarly for corner wch is shared by 8 cubes and for centre its 1...rest can be done urslf...idea is this.....