The friend drives 40*3=<<40*3=120>>120 miles.
Thus, Christina drives 210-120 =<<210-120=90>>90 miles.
Since the speed limit is 30 miles per hour when Christina is driving, she drives for 90/30=<<90/30=3>>3 hours.
Thus, she drives for 3*60=<<3*60=180>>180 minutes.
#### 180