The formula of an area of a tringle:
[tex]A_\Delta=\dfrac{base\cdot height}{2}[/tex]
We have:
[tex]b_1=20\ ft,\ h_1=12\ ft[/tex]
and
[tex]b_2=30\ ft,\ h_2=h[/tex]
The area does not change. Therefore we have the equation:
[tex]\dfrac{(30)(h)}{2}=\dfrac{(20)(12)}{2}\\\\15h=(10)(12)\\\\15h=120\qquad|:15\\\\\boxed{h=8\ ft}[/tex]
