In-class question 2 - Elevation from pressure measurements

Given that the sea level pressure is 1000 hPa, and the pressure on a mountain slope is 900 hPa, and the average temperature between 1000 hPa and 900 hPa is 20 degrees C; what is the elevation on the mountain slope? (We know that g = 9.8m s^-2 and R_d = 287Jkg^-1K^-1 )

Answer