COMEDK Exam 2015 Solved Paper

Surface area of bubble of radius, r=4πr2
Surface area of bubble of radius, 3r=4π(3r)2=36πr2
Therefore, increase in surface area
=36πr2−4πr2=32πr2
Since, a bubble has two surfaces, the total increase in surface area =2×32πr2=64πr2
∴ Energy spent = Work done = Surface tension × Total increase in surface area =64πσr2