The first one runs a mile in 7 minutes because 21 / 3 = <<21/3=7>>7
The second runs a mile in 8 minutes because 24 / 3 = <<24/3=8>>8
The first one would finish 5 miles in 35 minutes because 5 x 7 = <<5*7=35>>35
The second one would finish in 40 minutes because 5 x 8 = <<5*8=40>>40
It will take 75 minutes in total because 35 + 40 = <<35+40=75>>75
#### 75