Rania is riding the ferris wheel. Her vertical height $H(t)$ (in $\text{m}$) off the ground as a function of time $t$ (in seconds) can be modeled by a sinusoidal expression of the form $a\cdot\cos(b\cdot t)+d$. At $t=0$, when she starts moving, she is at a height of $10\text{ m}$ off the ground, which is as low as she goes. After $20\pi$ seconds, she reaches her maximum height of $30\text{ m}$. **Find $H(t)$.** *$\textit{t}$ should be in radians.* $H(t) = $ [[☃ expression 1]]