Essential University Physics: Volume 1 (4th Edition)

First, we convert rotations per minute to radians per second. This gives that the object is rotating at $1.05 \ rads/s$. Thus, we find: $\Delta t = \frac{ \Delta \omega}{\alpha} = \frac{\Delta \omega \times MR}{4F} = \frac{ 1.05 \times .7 \times 940} {4\times 20} \approx 9 \ s$