Linac Shielding Barriers Practice Problem

A linear accelerator for radiation therapy has a workload of 470 Gy/wk and is directed at a barrier for 27% of the time it is on. The room directly behind the barrier is uncontrolled and is occupied 67% 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 7.4 meters?