We could use the change of pressure to calculate for the height climbed by the mountain hiker. The change of pressure is given by
p = rho g h, where p is the change of pressure, rho is the air density, g is the acceleration due to gravity, and h is the height.
Using the conversion 1 mbar = 100 Pa,
(930 - 780)(100) = (1.20)(9.80)h
15000 = 1.209.80h
h = 1.28 km