Correct Answer - C
Applying, `(T_(1)-T_(2))/t = alpha[(T_(1)-T_(2))/2-T_(0)]` two lines we have,
or `(50-40)/5 = alpha[(50+40)/2 - 20]`.............(i)
and `(40-30)/t = alpha[(40+30)/2-20]`..............(ii)
Solving these two equations, we get
`t = 25/3min`