For water at 0ºC it takes one hour for first 3 mm of ice layer formation. How much time will it take for next 6 mm of ice formation?

time for ice formation is proportional to Xf2-Xi2 where Xf  is the final thickness and Xi  is the initial thickness t1t2=32-0292-32=18   t1= 1  s0 t2= 8 hours