How to CALCULATE in SECONDS Without a Calculator - Mental Math PART 2, by StickStories on YouTube. 8 min 24 s.
- Video length
- 8 min 24 s
- Time to do it
- 10 minutes
- Difficulty
- Easy
- Steps
- 8
What you need
Steps
Start with a warm-up problem to see your baseline speed, such as 46 × 68. Time yourself and write down the answer before learning any tricks.
Divide by 5 the easy way: divide the number by 10 first, then double the result. So 340 ÷ 5 becomes 34 × 2 = 68.
Add faster by rounding one number up to a round ten or hundred, adding, then taking back the extra. For 198 + 47, do 200 + 47 = 247, then subtract 2 to get 245.
Split percentages into 10% and 5% chunks. For 15% of 60, take 10% (6) plus 5% (3) to get 9.
Use the swap rule for awkward percentages: 8% of 50 is the same as 50% of 8, which is 4.
Multiply two-digit numbers by breaking one apart. For 46 × 68, do 46 × 70 = 3220, then subtract 46 × 2 = 92 to get 3128.
Check your answer with a rough estimate. 46 × 68 is close to 50 × 70 = 3500, so 3128 is in the right range.
Repeat the opening problem and time yourself again. Practise a handful of sums like this each day to build speed.
Tips and common mistakes
Say the intermediate numbers in your head or out loud at first, then drop the habit once the pattern feels automatic.
Always round to the nearest ten or hundred first, then adjust at the end, so you never lose track of the correction.
Keep a running total as you work left to right instead of holding several numbers at once.
Practise for 5 to 10 minutes daily rather than one long session, since speed comes from repetition.
Questions
How long does it take to get faster at mental math?
Most people notice a difference after a couple of weeks of short daily practice. Speed comes from repetition, not from long study sessions.
What is the quickest way to divide a number by 5?
Divide by 10 and then double the answer. For example, 85 ÷ 5 becomes 8.5 × 2 = 17.
Do these tricks work with large numbers?
Yes, the same rounding, splitting and percentage rules apply. With very large numbers, break them into smaller parts first so you can hold each piece in your head.
This guide was drafted from the video's details and general task knowledge. It has not been checked against the full video. Check the video and relevant product instructions before following a procedure. Report a correction.
