Silly Little Trick

I had to ponder this a little while. The trick as to why it works is to realise that 96 and 1930 give the current year (2026) when added together. Let's say you are \(x\) years old. We can form the following equation:$$ \begin{align} 96 - x + 1930 &=  \text{ Year you were born} \\ 2026 - x&=2026 - x \end{align}$$Any combination of two numbers that add to 2026 will work e.g. 100 and 1926. This combination however, would immediately draw suspicion because it's easy to see that the two add to 2026. However, the problem should read:

• A book has 96 pages 

• Subtract the age you will turn this year 

• Add the result to 1930 

• You will find the year you were born

Next year the book would need to have 97 pages and so on. I asked Gemini to create a similar problem:

Can you propose another age related trick mathematical question in the spirit of the one shown in the image but different and not easy to spot the mathematical trickery behind it. 

Here's what it came up with:

 Try this sequence to find someone's birth year:

• Take the number of months in a year (12).

• Multiply it by the number of days in a standard week (7).

• Multiply that result by the number of hours in a day (24).

• Add the total number of fingers on your hands (10).

• Subtract your current age.

The final number will be the exact year you were born.

The Mathematical Breakdown

All age-calculating tricks ultimately rely on the same fundamental algebraic formula: $$\text{Current Year} - \text{Age} = \text{Birth Year}$$The key to making the trick impressive is how well you hide the current year from the participant.

The example shown in the above image thinly veils this formula by splitting the current year (2026) into two arbitrary numbers: 96 and 1930. The instructions ask you to subtract your age from the page count (96) and then add 1930. Mathematically, this is simply \( (96 + 1930) - \text{Age}\), which immediately resolves to \(2026 - \text{Age}\). The trickery is easy to spot because the numbers are combined using basic addition, and 1930 already looks suspiciously like a year.

The proposed trick above is much harder to spot because it obfuscates the constant (2026) by burying it at the end of a multi-step multiplication chain. It also uses a psychological misdirection: by asking the person to multiply time-related trivia (months, weeks, days, hours), they assume the math has something to do with calculating the lifespan in hours or days.

Here is how the hidden constant is built:$$\begin{align} 12 \text{ (months)} \times 7 \text{ (days)} &= 84\\84 \times 24 \text{ (hours)} &= 2016 \\ 2016 + 10 \text{ (fingers)} &= 2026 \end{align} $$By the time the participant calculates 2026, their working memory is occupied by the multiplication steps rather than recognizing that they have just arrived at the current year. Subtracting their age from this stealthily generated 2026 yields their birth year perfectly. (Note: As with all standard age tricks, this assumes the participant has already celebrated their birthday in the current calendar year; otherwise, the result will be off by one).