As per the question requirement for both to be equally efficient
So, after t=3.33 both will be equally efficient.
Now we would consider individually
% autoclave after 6 min= 70-6"\\times2=70-12=58 %" %
and for dry heat asfter 6 min = "80 -5\\times6=50" %
so here difference will be = 58-50 = 8 %