Learn the patterns
Mental math tricks you can actually reuse.
Each guide explains why the shortcut works, when to use it and how to check the result. Try the same pattern with your own numbers in CalcTrick.
Quarter of 100
How to multiply by 25 mentally
Multiply by 25 in your head by dividing the other factor by 4 and multiplying by 100, with clear worked examples.
n × 25 = (n ÷ 4) × 100
Ten copies plus one
How to multiply by 11 mentally
Learn a reliable mental method for multiplying any number by 11 by adding one ten-times copy and one original copy.
n × 11 = (n × 10) + n
Double, then divide by 10
How to divide by 5 mentally
Divide by 5 quickly by doubling the number and then dividing by 10, with practical examples and decimal results.
n ÷ 5 = (n × 2) ÷ 10
One less than 100
How to multiply by 99 mentally
Multiply by 99 by multiplying by 100 first and subtracting one original copy, with step-by-step examples.
n × 99 = (n × 100) − n
Your turn
Find a simpler route for your calculation.