The first plane goes 100 MPH slower than empty because 50 x 2 = <<50*2=100>>100
The first plane goes 500 MPH because 600 - 100 = <<600-100=500>>500
The second plane goes 120 MPH slower because 60 x 2 = <<60*2=120>>120
The second plane goes 480 because 600 - 120 = <<600-120=480>>480
The third plane goes 80 MPH slower because 40 x 2 = <<40*2=80>>80
The third plane goes 520 MPH because 600 - 80 = 520
The total speed is 1500 because 500 + 480 + 520 = <<500+480+520=1500>>1500
Their average speed is 500 because 1500 / 3 = <<1500/3=500>>500
#### 500