Answer
The roller coaster should be moving at a minimum speed of $17.0~m/s$
Work Step by Step
To find the minimum required speed at the top of the loop, we can assume that the gravitational force provides the centripetal force at the top of the loop. That is we can set the radial acceleration equal to $g$:
$\frac{v^2}{r} = g$
$v = \sqrt{g~r}$
$v = \sqrt{(9.80~m/s^2)(29.5~m)}$
$v = 17.0~m/s$
The roller coaster should be moving at a minimum speed of $17.0~m/s$
