Safety regulations require that the time between airplane takeoffs (on the same runway) will be at least 2 minutes. When taking off, the run time of an airplane on the runway is 42 seconds. Planes are on average waiting 3 minutes and 42 seconds for take-off. On average there are 15 planes taking off per hour. Required:
How many planes are either on the runway or waiting to take off?

1.03 airplanes

Explanation:

Calculation for How many planes are either on the runway or waiting to take off

First step is to calculate the flow time

Flow Time = (0.70 + 3.42)/60

Flow Time =4.12/60

Flow Time= 0.0687

Last step is to calculate How many planes are either on the runway or waiting to take off

Using this formula

Numbers of plane = Throughput Rate × Flow Time

Where,

Throughput Rate = 15 planes per hour

Let plug in the formula

Numbers of plane= 15 × 0.0687= 1.03 airplanes

Note that runway 42 seconds/60 minutes will gives us 0.70

Therefore How many planes are either on the runway or waiting to take off will be 1.03 airplanes

you have to put the parentheses like this:

in fact, you have to perform the operation inside parentheses first, and then proceed in order: