Computerwarehouse.com charges $8 to ship orders up to 10 lb, $12 for orders from 10 lb up to 15 lb, and $15 for orders of 15 lb or more. One day, shipping charges for 150 orders totaled $1620. The number of orders under 10 lb was three times the number of orders weighing 15 lb or more. Find the number of packages shipped at each rate.