🧮 Mental Math
The everyday arithmetic you actually use: adding, multiplying, percentages, squares. None of it takes talent, just the right shortcut and enough reps to make it automatic. Pick a drill and start stacking them up.
Addition & Subtraction Drills →
Work left to right: add the tens first, then the ones, then combine. For subtraction, it is usually faster to count up than to take away.
Multiplication Drills →
Break the second number into parts you already know, multiply each part, then add. Rounding to a nearby ten and correcting also works well.
Percentage Calculation →
Start from 10% — it is just the number with the decimal moved one place. Build every other percentage from 10%, 5% and 1% chunks.
Averages & Means →
The average is the total divided by the count. When numbers are close together, pick a base value and average only the differences from it.
Two-Digit Multiplication →
Keep the first number whole and split the second into its tens and ones. Multiply the whole number by each part, then add the two results.
Multiply by 11 Trick →
Multiplying by 11 is the same as multiplying by 10 and adding the number once more. For two-digit numbers you can also spread the digits and add the middle.
Squaring Two-Digit Numbers →
For a number ending in 5, multiply the tens digit by the next number up and stick 25 on the end. For others, round to a nearby ten and adjust.
Adding Large Numbers →
Add from the biggest place value down: thousands, then hundreds, then the rest. Keep one running total rather than stacking columns in your mind.
Tip & Bill Percentages →
Find 10% of the bill first, then scale it. A 20% tip is double the 10%; 15% is the 10% plus half of it again.
Fractions to Percentages →
Learn the handful of building blocks: 1/2 is 50%, 1/4 is 25%, 1/5 is 20%, 1/8 is 12.5%. Most fractions are just multiples of these.