Isolation of Elements

Galena (an ore) is partially oxidized by passing air through it at high temperature. After some time, the passage of air is stopped, but the heating is continued in a closed furnance such that the contents undergo self-reduction. The weight (in kg) of Pb produced per kg of O2 consumed is ________.
(Atomic weights in g mol−1: O = 16,S = 32,Pb = 207)