Hi I am trying to explain to my children how this problem works.  It was sent to me on the internet and I can not figure it out. They keep asking me how it works and I can not tell them.
 
The problem is:
 
“ I can Guess your birthday”:

1. Multiply the number of the month in which you were born by 5.
2. Add 17.
3. Double the answer.
4. Subtract 13.
5. Multiply by 5
6. Subtract 8.
7. Double the answer.
8. Add 9
9. Add the number of the day on which you were born.
10. Subtract 203.

Thanks so much,
 
Michelle

Hi Michell,

Let me to try to explain how it works with a specific example. I am going to use the month as 8 and the day as 12 and go through the steps, but not perform all the arithmetic.

1. 5 8
2. 5 8 + 17
3. 2(5 8 + 17) = 10 8 + 34
4. 10 8 + 34 - 13 = 10 8 + 21
5. 5(10 8 + 21) = 50 8 + 105
6. 50 8 + 105 - 8 = 50 8 + 97
7. 2(50 8 + 97) = 100 8 + 194
8. 100 8 + 194 + 9 = 100 8 + 203
9. 100 8 + 203 + 12
10. 100 8 + 203 + 12 - 203 = 100 8 + 12

Now see what you have. There is the number of the month (it is 8 here but it could be any month) followed by two zeros, plus the number of the day (12). So for this example I get 800 + 12 = 812.

I don't know how old your children are, but steps 3, 5 and 7 may need some explanation, I would try it this way even though it's not great. Suppose that I have five eight-dollar bills in my left pocket and a seventeen-dollar bill in my right pocket. If somehow I double my money then I have ten eight-dollar bills in my left pocket and two seventeen-dollar bills in my right pocket. That is 10 $8 + $34.

Penny

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