Arms and Legs
Explanation:By inspection
From the diagram it is Legs and Arms
By calculation
It is a matter which of the three will accumulate to 23of an hour
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Neck
Summing 12's only gives integer values giving 1,2,3,4... or
integer values + 12 and 0 +12 giving 12,112,212,...
So neck is excluded
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Legs
Every 13:⇒13+12=23
Legs will be stretched at 23 hour
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Arms
Every 16:=>16+16+16+16=46
Divide top and bottom by 2 giving:
4÷26÷2≡23
Arms will be stretched at 23 hour