Think clearly.
Calculate faster.
Every trick includes its rule, reasoning, conditions, exceptions and checked examples—so speed never replaces understanding.
Multiply a two-digit number by 11
Add its digits and insert the sum between them, carrying when needed.
Learn this trick →Multiply by 125
Divide by 8 and multiply by 1000.
Learn this trick →Multiply by 25
Divide the other number by 4, then multiply by 100.
Learn this trick →Multiply by 5
Multiply by 10, then halve the result.
Learn this trick →Multiply by 9
Multiply by 10 and subtract the original number.
Learn this trick →Multiply by 99
Multiply by 100 and subtract the original number.
Learn this trick →Multiply numbers equally spaced around a centre
Multiply (a-b)(a+b) by calculating a² - b².
Learn this trick →Square numbers ending in 5
Multiply the leading part by its next integer, then append 25.
Learn this trick →Square numbers near 100
Write the number as 100 + d, then calculate 10000 + 200d + d².
Learn this trick →Combine two successive discounts
For discounts a% and b%, the effective discount is a + b - ab/100 percent.
Learn this trick →Find 12.5 percent by dividing by 8
Divide the quantity by 8.
Learn this trick →Find 15 percent quickly
Add 10% of the quantity to 5% of it.
Learn this trick →Swap percentage values
x% of y equals y% of x.
Learn this trick →Divisibility by 11
Subtract the sums of alternate digits. A multiple of 11, including zero, means the integer is divisible by 11.
Learn this trick →Divisibility by 3
Add the digits; the integer is divisible by 3 when its digit sum is.
Learn this trick →Divisibility by 9
Add the digits. If the sum is divisible by 9, the number is divisible by 9.
Learn this trick →