Magic trick

K

Kaycee

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Aug 26, 2005
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Can you please help me with this question. I am having trouble with the question about place value. Thank you.



One of Dr. Wonderfuls other mind- reading tricks involves birthdays. Use a calculator and follow along with the crowd as he gives the directions. Press + on your calculator after each step.

1. Enter the month of your birthday.

2. Multiply by 5.

3. Add 20.

4. Multiply by 4.

5. Subtract 7.

6. Multiply by 5.

7. Add the day of your birthday.

8. Subtract the number of days in a non-leap year.


To the right of each step above, write an algebraic expression that correctly descripes Dr. Wonderful's direction. Study the sequence of expressions and then explain how place value helps to explain how this "magic number trick" works.
 
Call the month M and the day D.
Follow the instructions.
5M+20
20M+80
20M+73
100M+365+D
100M+D
 
I've seen people thoroughly overwhelmed by such trivial demonstrations. Often, I hear comments such as, "Wow! Who figured this out? He must be a genius!" Generally, it is first year algebra, the stuff 7th or 8th graders might find mystifying.

Think "algebra" and demystify it in your mind, as Gene has demonstrated. One can think these up all day long. There is absolutely NOTHING magic or tricky about them. Simple algebra.

Don't feel bad. Many have fallen for it.
 
Ya...and that one is sure easy to "see":
look at it carefully, Kaycee, and you'll see that the steps
force the initial month to end up multiplied by 100 (so 800 if August)
to which the day is added (800 + 17 = 817 if 17th of August)...
 
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