A composite string is made up by joining two strings of different masses per unit length $\mu$ and $4\mu$ respectively. The composite string is under the same tension. A transverse wave pulse $Y = (6\,\text{mm})\sin(5t + 40x)$, where $t$ is in seconds and $x$ in metres, is sent along the lighter string towards the joint. The joint is at $x = 0$. The equation of the wave pulse reflected from the joint is