Looking for a quick math warm-up that gets students excited while sneaking in essential operations practice? This classic “mind reader” math trick is a student favorite.
No matter what starting number your students choose—whether it’s 2, 75, or 10,000—the final answer will always be 5.
It’s completely self-checking: if a student ends up with any answer other than 5, they know a calculation error occurred along the way and can retrace their steps!
Have your students follow these five simple steps on a piece of paper or individual whiteboards:
Pick any number. (Example: 100)
Double it. (100 X 2 = 200)
Add 10. (200 + 10 = 210)
Halve the result. (210/2 = 105)
Subtract your starting number. (105 – 100 = 5)
This activity is remarkably versatile across upper primary and middle school grades:
Math Warm-Up / Brain Break: Run through the steps together on the board to get brains moving before starting your core lesson.
Self-Checking Practice: Encourage students to test the trick with multi-digit numbers, decimals, or fractions to challenge their computational speed.
The “How It Works” Extension: Challenge advanced thinkers or early finishers to figure out why the answer is always 5.
For older students ready for an introduction to algebra, you can break down the equation together on the board:
Let the starting number be n.
Double it: 2n
Add 10: 2n + 10
Divide by 2: {2n + 10}/{2} = n + 5
Subtract original number n: (n + 5) – n = 5
By showing students that halving the added number (10 divided by 2) is the secret behind the result, you can encourage them to invent their own “Always 3” or “Always 10” math tricks!
