computerwarehouse charges $3 for shipping orders up to 10 ib, $5 for orders from 10 ib up to 15ib, and $7.50 for orders of 15ib or more. one day shipping charges for 150 orders totaled $680. the number of orders under 10 ib was three times the number of orders weighing 15 ib or more. find the number of packages shipped at each rate.