At an average of 30mph, the first mile took 2 minutes (120 seconds). At an average of 90mph, the second mile took 40 seconds. So your average speed was 2 mile

At an average of 30mph, the first mile took 2 minutes (120 seconds). At an average of 90mph, the second mile took 40 seconds. So your average speed was 2 miles / 160 seconds, or 45 mph.