A 15m rope is fixed to a rigid wall at one end. The first 9.0m of the rope (measured from the free end) has a linear mass density of 4.0×10−3kgm−1; the remaining 6.0m next to the wall has a linear mass density of 2.0×10−3kgm−1. The tension in the rope is kept at 50N throughout. A short upward pulse is generated at the free end and travels toward the wall, reflects from the wall (treated as a fixed end), and returns to the starting point. Which time interval is closest to the total elapsed time from launch to return?
(a) 0.18s
(b) 0.24s
(c) 0.12s
(d) 0.27s