Here, we're looking for a circumference value, but just the portion that the minute hand travels.
Step 1) Find the portion of the circumference that the minute hand travels
108/360 = 0.3
Step 2) Find the circumference
-Remember that the diameter is double that of the radius!
C = pi x 20 = 20pi cm
Step 3) Use the circumference and portion that the minute hand travels to find the length the minute hand travels
20pi x 0.3 = 18.8 cm
Hope this helps! :)