Mental math tricks

Mental math is faster with a few techniques: make a ten, round and then adjust, split a product into tens and ones, and build any percent from 10%, 5% and 1%. These 13 are the quick techniques from Doing Math’s lessons, each with its steps, a worked example and three problems to try.

• Make tenA sum crosses a ten, a hundred or a thousand.
• Doubles and near doublesThe two numbers are the same or close together.
• Count up to subtractSubtracting, making change, or finding a gap.
• Round, then adjustA number sits just below a round one, like 29, 98 or $1.99.
• Split and combineMultiplying a two-digit number by a one-digit number.
• Double and halveOne number is even and the other ends in 5.
• Shift for 10, 100 and 1,000Multiplying or dividing by 10, 100 or 1,000.
• Build from 10%, 5% and 1%Finding any percent of an amount.
• Know the equivalentsA percent is a simple fraction, or a fraction is a simple percent.
• Estimate firstBefore a long calculation, and to check an answer.
• Work backward from a percentYou know the amount after a percent change and want the amount before it.
• Pretend it is 100A percent question gives no amounts at all.
• Keep the balanceSolving any equation.

Then put them to work against the clock: the mental math speed drill.