For this problem , why we dont have to consider the max shear occur under the concentrated load just like the case in max shear act on point ? In the previous question , the author 'move the load' so that the the center of the beam is between the FR (resultant force ) and the load ... Why in this case , we just need to consider 2 supports only ? Or the question is wrong ?
This question is looking for the absolute largest shear, the previous question was looking for the largest shear at midspan.
If you draw the shear force diagram for a simply supported beam, subjected to loads in one direction only, you will notice that the largest shear is always at one of the supports. Hence there was no need to consider anything other than the supports in this question.
As a note: in my opinion the author of this question missed one possibility they should have considered: what happens if you move the load far enough to the left that the 5kN load drops off the beam, and move the first 20kN load to be directly over A? Answer: you get a shear at A of 60.5kN. Still not quite as big as found by the author at B, but bigger than what they found at A! Of course, this only applies if its possible to move the loads off the beam. As I normally design bridges, I expect loads to be able to move off the end.