secrets-of-mental-math-malestrom

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Secrets of Mental Math
Why This Trick Works
Look at the numbers in each set of cards and see if you can find
anything consistent about them. Each set of numbers sums to
the same total. Set A numbers total to 20. Set B numbers total
to 23. Set C numbers total to 17. With person C’s numbers in
the right column totaling to 17, you will always put down the 7
and carry the 1. With person B’s numbers totaling to 23, plus
the 1, you will always put down the 4 and carry the 2. Finally
with person A’s numbers totaling to 20, adding the 2 gives you
a total of 2247!
A DAY FOR ANY DATE
We conclude our book with one of the classic feats of mental
calculation: how to figure out the day of the week of anyone’s
birthday. This is actually a very practical skill. It’s not every day
that someone will ask you to square a three-digit number, but
hardly a day goes by without somebody mentioning a date to
you in the past or future. With just a little bit of practice, you
can quickly and easily determine the day of the week of practically
any date in history.
First we assign a code number to every day of the week. They
are easy to remember:
Number
Day
1 Monday
2 Tuesday
3 Wednesday
4 Thursday
5 Friday
6 Saturday
7 or 0 Sunday