If the total amount of physical copies sold is 10,000,000 then:
0.14x10,000,000=1,400,000 is the amount of PC physical copies.
If it is true that PC physical copies represent 20% of PC total sales:
1,400,000/0.20=7,000,000 total PC sales (both downloaded and physical copies).
XBox 360 (physical) sales are 59% of the total physical e.g.
0.59x10,000,000=5,900,000 units
There are ~19% more PC players than Xbox 360 players, unless Skyrim can be downloaded through Xbox LIVE and a significant amount of buyers acquired the game that way. Take that as you will.