They ran 2,400 meters because 24 laps x 100 m/lap = <<24*100=2400>>2400 m
They ran 200 meters per minute because 2400 m / 12 min = <<2400/12=200>>200 m/min
For every minute they ran they earned an average of $7 because $3.5/100m x 200 m/min = $<<3.5/100*200=7>>7/min
#### 7