Answer:
Step-by-step explanation:
Let t = flying time in seconds for each plane
Descending rate = 40 ft/s
Ascending rate = 60 ft/s
Descending height, hd= 19200 - 40t
Ascending height, ha = 60t
Equating hd = ha, therefore:
60t = 19200-40t
60t + 40t = 19200
100t = 19200
t = 19200/100
t = 192 seconds
h = 19200 - 192(40)
h = 19200 - 7680
= 11,520 ft.