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#4 Quick Addition with the Calendar

"I can add very fast," boasted
Billy. "Here, I'll show you. In this calendar, I want you to choose some full
week of 7 days. It can be any wekk in any month as long as it has 7 days. Now add
the numbers in that week. But don't tell me the sum. Just tell me the number
of the first day of the week, and I'll find the sum, too."


Mary chose the week of April 8. Shee
added the numbers very carefully. (Did she get 77?)


"I'm ready," said Mary.
"I chose the week in April beginning on the eighth."


"Then your sum is 77," said Billy
after a breif moment.


"That's right, Billy. How did you
add it so fast?"

The
key is add 3 to the first day of the week. Then multiply the sum by 7.
Think about why this works and where else you
can use it.
Hey Don't forget to check out the other Number tricks...
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