Two machines produce the same item. Machine A can produce 15 items per/hr and Machine B can produce 10/hr. At least 420 of the items must be produced each 40-hr. week, but the machines cannot be operated at the same time. If it costs $50 per hour to operate A and $30 per hour to operate B, determine how many hours per week to operate each machine to meet the production requirement at minimum machine cost.