Linac Shielding Barriers Practice Problem

A linear accelerator for radiation therapy has a workload of 411 Gy/wk and is directed at a barrier for 21% of the time it is on. The room directly behind the barrier is uncontrolled and is occupied 58% of the time. What is the number of tenth value layers (TVL) required to reduce the primary radiation dose to the acceptable level at a distance of 8.5 meters?