Answer:
1 plane.
Step-by-step explanation:
Let's call the number of planes waiting or on the runway of WIP, the number of planes taking off per hour of TR, and the time for waiting and the runway of TT, so:
WIP = TRxTT
TR = 18 airplanes per hour
Knowing that 1 min = 60s ⇒ 39s = 39/60 = 0.65 min, and 5 min and 33s = 5 + 33/60 = 5.55 min
TT = 0.65 + 5.55 = 6.2 min/60 = 0.1033 h
WIP = 15x0.1033 = 1.55
So, there is 1 plane either on the runway or waiting to take off