Answer
The time to reach the bottom is the same for both the marble and the cannonball.
Work Step by Step
We can write an expression for the rotational inertia of a solid sphere:
$I = \frac{2}{5}MR^2$
We can use Equation (11-10) to find the acceleration of a solid sphere down the incline:
$a = \frac{g~sin~\theta}{1+I/MR^2}$
$a = \frac{g~sin~\theta}{1+\frac{2}{5}MR^2/MR^2}$
$a = \frac{g~sin~\theta}{1+\frac{2}{5}}$
$a = \frac{5}{7}~g~sin~\theta$
Since the acceleration of the marble and the cannonball is the same, the time to reach the bottom is the same for both the marble and the cannonball.
