The above problem was asked in GATE(Graduate Aptitude Test in Engg) 2015 in India.
I have no idea where to start so I can't show my attempt. But through google, I got solution by 2 different coaching institutes
IIT(Indian Institute of Technology), the governing body of exam says that the answer will be in the range 60-64V
Please help me with the problem, as similar problems will be asked in the exam.