The potential energy of a certain spring when stretched through a distance ‘S’ is 10 joule. The amount of work (in joule) that must be done on this spring to stretch it through an additional distance ‘S’ will be
12kS2 = 10 J (given in the problem)
12k [(2S)2−(S)2] = 3×12kS2 = 3×10 = 30 J