Imagine two circles with equal arcs AB and PQ, try to superimpose the two circles such that A coincides with P and B coincides with Q then you can see that chords AB and PQ are equal.
Rotate fig 1 and superimpose it on fig 2 such A coincides with P and B coincides with Q. you can find that lengths of chord AB and PQ are equal.
PQ = 60cm