Answer:
The answer to both parts is (C)
In the first 9/3.9 = 2.31 half lives elapsed
(1/2)^2.31 = .5^2.31 = .202 pressure at 9 mi above sea level
P = .202 * 31 = 6.26 mi
In the second:
19/5 = 3.8 half-lives
.5^3.8 = .072 left after 3.8 half lives
635.8 * .072 = 45.6