This will work for anyone whose age is between one and ninety-nine. I am going to ask you to do a little arithmetic. My calculator will use the results to display your age and the number of your house. You will need to perform these calculations:

1. Enter your house number.
2. Double it.
3. Add 42.
4. Multiply the answer by 50.
5. Subtract the year of your birth.
6. Subtract 50.
7. Add the number of birthdays you have had so far this year, i.e., either 0 or 1.
8. Subtract 28.

I now predict that the last two digits of the result will be your age, and the remaining digits will be the number of your house.

Let’s try it on Alice, who lives in a house whose number is 327. She was born on October 31, 1991, and she is 28 years old. She’ll be 29 on October 31, 2020.

• Enter your house number: 327.
• Double it: 2*327 = 654
• Add 42: 654 + 42 = 696.
• Multiply by 50: 696*50 = 34,800
• Subtract the year of your birth: 34,800 – 1991 = 32,809
• Subtract 50: 32,809 – 50 = 32,759
• Add the number of birthdays you have had so far this year: 32,759 + 0
• Subtract 28: 32,759 – 28 = 32,728

So the house number is 327, and Alice is 28 years old!

Why does this work? Let’s do it by calling your house number H and the year you were born Y.

• Enter your house number: H
• Double it: 2*H
• Add 42: 2*H + 42
• Multiply by 50: 50*(2*H + 42) = 100*H + 2100
• Subtract the year of your birth (call it Y): 100*H + 2100 – Y
• Subtract 50: 100*H + 2100 – Y – 50 = 100*H + 2050 – Y
• Add the number of birthdays you have had so far this year: 100*H + 2050 – Y (+ 0 or 1)
• Subtract 28: 100*H + 2019 (+ 0 or 1): 100*H + 2019 (+ 0 or 1) – Y

When you subtract your age, Y, from 2019 (+0 or 1), you will get a 2-digit number which is your age. (If you are 9 or younger, the first digit is 0; if you are over 99, this won’t work!)

The remaining digits are your house number!

No mystery!