Equal masses of lead 207Pb and silicon 28Si are cooled using liquid helium (the boiling point of helium is 4.2 K at normal pressure) from temperature T1 = 20 K to T2 = 4.2 K. Estimate the ratio of the masses of liquid helium required for cooling lead and silicon, if it is known that the Debye temperatures are equal: Q(Pb) = 95 K and Q(Si) = 645 K. Neglect the heat capacity of the electrons.

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