 Quandaries and Queries 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”: Multiply the number of the month in which you were born by 5. Add 17. Double the answer. Subtract 13. Multiply by 5 Subtract 8. Double the answer. Add 9 Add the number of the day on which you were born. 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. 5 8 5 8 + 17 2(5 8 + 17) = 10 8 + 34 10 8 + 34 - 13 = 10 8 + 21 5(10 8 + 21) = 50 8 + 105 50 8 + 105 - 8 = 50 8 + 97 2(50 8 + 97) = 100 8 + 194 100 8 + 194 + 9 = 100 8 + 203 100 8 + 203 + 12 100 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 Go to Math Central