Answer
5.03 cm
Work Step by Step
Radius $r=1.2\,cm$
In order to complete $360^{\circ}$ the minute hand takes 60 minutes.
Therefore in 40 minutes it covers an angle of $360^{\circ}\times\frac{40}{60}=240^{\circ}$.
That is, central angle $\theta=240^{\circ}=(240\times\frac{\pi}{180})\,radians=\frac{4\pi}{3}\,rad$
Arc length $s=r\times\theta=1.2\,cm\times\frac{4\pi}{3}=5.03\,cm$