Mental math tricks for everyday life

Splitting a bill, working out a discount, doubling a recipe. Everyday math is rarely hard, but it can feel stressful when someone is waiting. A few habits make it quicker and calmer.

Round first, then adjust

Most everyday math doesn't need to be exact right away. To add $19.95 and $42.10, think $20 plus $42, which is $62, then adjust: a little more, about $62.05. Rounding gives you a quick, close answer, and you can fine-tune it if you need to.

Break numbers apart

Multiplying 7 by 18 in your head is easier as 7 times 10 plus 7 times 8: 70 plus 56 is 126. Splitting a number into friendly parts (tens and ones, or halves) turns one hard step into two easy ones.

Percentages: start from 10%

To find 10% of any amount, move the decimal point one place left: 10% of $64 is $6.40. From there:

• 5% is half of 10%: $3.20.
• 20% is double 10%: $12.80.
• 15% is 10% plus 5%: $9.60.

This covers most tips and discounts.

Fractions in the kitchen

Scaling a recipe is mostly halves, doubles and thirds. To make 1.5 times a recipe, work out half of each amount and add it on. Half of 3/4 cup is 3/8 cup, which is 6 tablespoons, or a little more than a third of a cup.

Check if it's reasonable

After any mental calculation, ask: does this answer make sense? If 15% of a $40 bill comes out as $60, something slipped. A quick sense check catches most mistakes.

Practice it

Think Sharp's math games practice these everyday moves. Number Rain asks you to solve problems quickly before they reach the water. Recipe Scale turns fractions into marks on a measuring tool. Bigger Side compares two amounts, and Trade Up asks which offer is worth more.

Then try one in real life: next time you eat out, work out the tip before you look at the suggested amounts.