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Keep constructing triangles in the incircle of the previous triangle. What happens?

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Vedic Sutra is one of many ancient Indian sutras which involves a cross subtraction method. Can you give a good explanation of WHY it works?

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Zeller's Birthday

Age 14 to 16 Challenge Level:

The calculation must 'count back', or 'count forward', from somewhere in some way.

First in whole weeks, then the bit that is left (the days that don't make a whole week) could indicate the position in the week, ie. 'day'