A worker can assemble 3 shelf units each hour. During the fifth hour of the shift, there were 115 shelf units in the warehouse. How many units were assembled before the shift started?

A worker can assemble 3 shelf units each hr. During the 5th hr ... so the shift has been going on for 5 hrs ... at 3 units per hr = (3 * 5) = 15 units assembled during the shift ... and if there was 115 units in the warehouse, then that means before the shift, there were (115 - 15) = 100 units assembled <=