How do I solve this?

Question

The usual partial pressure of oxygen that people get at sea level is .20 atm, that is, a fifth of the usual sea level air pressure. People used to 1 atm air pressure begin to become light headed at about .10 atm oxygen. As a rule of thumb, the air pressure decreases one inch of mercury each thousand feet of altitude above seal level. At what altitude should airplane cabins be pressurized? Up to about what altitude should you be able to use unpressurized pure oxygen? (Express your answer in feet above Mean Sea Level, or MSL.)

Answer 1

See your earlier posts.

Answer 2

All you wrote was see above.

Answer 3

I answered the FIRST time you posted and when you went back you didn't go back far enough. Here is the original post.
http://www.jiskha.com/display.cgi?id=1270011834