To get Delta Airline Miles, they must first, obviously, fly with Delta Airlines. Then they can redeem their Airline Miles at any Delta Airlines office. They will provide them with the tickets to then fly using the previous credits recieved.
The best way is to check with the Delta Sky Miles program itself. USA Today, also has a guide in their Travel Tips section with links to guides and charts.
Average speed = (delta-X) / (delta-T) = (10 miles) / (2 hours) = 5 miles per hour
As of my last update, Delta Air Lines has not publicly disclosed the exact number of three million milers in their SkyMiles program. However, it is known that achieving this status is quite rare and represents a significant level of loyalty and travel with the airline. Delta's loyalty program includes various tiers, with three million milers being one of the highest achievements for frequent flyers. For the most current statistics, checking Delta's official communications or reports would be advisable.
That varies, between one airline and another - or, within the same airline, between one year and another.
American Airlines sells as few as 1,000 miles for $27.50 and as many as 40,000 miles for $1,000 [source: American Airlines