An arc is a part of the circumference of a circle and the angles that subtend the arcs are proportional to the length of the arc.
So, you basically have two arcs to compare: a 10cm arc and a 60cm arc. But the 60cm arc also happens to be the circumference in this case, so you know that the angle that subtends that arc is 360 degrees (the whole circle). The 10cm arc is 1/6 of the whole circumference so the angle is going to be 1/6 of 360 degrees, which is 60 degrees.
