Learn · 3 min read
🧮 Mental Math Tricks: Multiply, Add and Subtract in Your Head
Mental math isn't about having a special brain. It's about knowing a few shortcuts that turn hard sums into easy ones. Each trick below takes a minute to learn and a few days of practice to become automatic.
Multiply by 5
Multiply by 10, then halve it. Ten times is easy (add a zero), and halving is easier than multiplying by 5.
Multiply by 9
Multiply by 10, then subtract the number once.
Tip: For the 9 times table up to 10, the digits of the answer always add up to 9: 18, 27, 36, 45…
Multiply a two-digit number by 11
Split the digits apart and put their sum in the middle. If the sum is 10 or more, carry the 1 to the first digit.
Square a number ending in 5
Take the digits before the 5, multiply that number by the next number up, and put 25 on the end.
Double and halve
If one number is even, halve it and double the other. The answer stays the same, and the sum often gets much easier.
Add and subtract by rounding
Round one number to something easy, do the sum, then fix the difference.
Subtract by counting up
To take a number away from 100 or 1000, count up from the smaller number to the bigger one in easy steps. This is how shopkeepers used to give change.
Percentages
- 10% means move the decimal point one place left: 10% of 240 is 24.
- 5% is half of 10%: 5% of 240 is 12. 15% is 10% + 5% = 36.
- x% of y is the same as y% of x — so 8% of 50 is the same as 50% of 8, which is 4.
Divisibility rules
- By 2: the last digit is even.
- By 3: the digits add up to a multiple of 3 (51 → 5 + 1 = 6, so yes).
- By 4: the last two digits make a multiple of 4 (316 → 16, so yes).
- By 5: it ends in 0 or 5.
- By 6: it passes the tests for both 2 and 3.
- By 9: the digits add up to a multiple of 9 (729 → 18, so yes).
Making it automatic
Pick one trick a week and use it whenever you see a chance. Speed comes from short, regular practice rather than long sessions — five minutes a day of quick sums builds more fluency than an hour once a week. Our Math and Flash games are built for exactly that kind of quick daily practice, with levels that grow with you.