On a stopwatch, the tip of the second hand moves 2 cm in 15 seconds. how long is the second hand (to the nearest tenth)?

The 'seconds' hand on the watch goes all the way around in 1 minute. 15 seconds is 1/4 of a complete 1-minute revolution. So, the 2 cm is 1/4 of the way around the circle that the tip makes. This makes the full circumference of the circle 8cm.

The length of the second hand will be the radius of the circle. Use the following equation: 

2 pi radius  =  circumference

Since we know the circumference, plug that in, and solve for the radius:

2 pi radius  =  8 cm              
radius  =  8cm / 2 pi  =  1.27 cm

Logical equivalent of the give question is its contrapositive