**Why does adding two numbers on a watch dial equal 13?**We see six pairs of such numbers on the clock. The explanation is very simple.

Why 13? |

If we add 1 to one term and subtract 1 from another term, the sum will remain the same. This can be repeated many times and the result of the addition will not change. This property of addition can be observed not only on the watch dial, but also on the number line.

Sum on a number line |

As the initial sum on the number line, you can take two adjacent numbers (top picture) or two numbers located next to the number we chose (bottom picture). I called such sums “Decomposition sums”.

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