General Question

seVen's avatar

What's your method of calculating fast in your head instead of using a calculator/paper & pen?

Asked by seVen (3472points) November 10th, 2008
Observing members: 0 Composing members: 0

10 Answers

Allie's avatar

Trying to break it down and find a part of the number I can calculate easily, then adding that to the rest of it. (Did that make sense?) Like if I want to multiply 24×8 I’ll do 8×20 + 8×4.

TheBox193's avatar

Here are 3 cool tips I just found:

1. Round Off to a Quick Approximation

First determine a level of accuracy you are happy with, then round the numbers appropriately before making the calculation. This is also useful to determine a quick ballpark figure before doing the actual calculation.

2. Break Up Into Parts

Say you have to multiply 613 by three. Try multiplying first 600 by 3, then 10 by 3 then 3 by 3 and adding the results.

4. Quickly Find Three Common Percentages

20% -> Divide the number by 5
33.3% -> Divide the number by 3
50% -> Divide the number by 2


jasongarrett's avatar

If I want to be correct, I use a calculator.

If I only want to be in the right ballpark, I’ll round all the numbers to the nearest power of 10 and then do the math. In Allie’s example, I’d round 24×8 to 10×10 and say the answer is “over a hundred.” This is TheBox193’s method #1 with a level of accuracy of “not very.”

If I want someone else to figure it out, I’ll declare some arbitrary number to be the answer until someone gets annoyed enough to prove me wrong.

Allie's avatar

jason: Wouldn’t it be more accurate to call 8 a 10 and 24 a 20? Then do 20×10 and say it’s around 200? Cause using 10×10 leaves you nearly 100 off.

…but I get what you’re saying.

jasongarrett's avatar

Yeah, the order-of-magnitude thing works better with bigger numbers.

psyla's avatar

I agree, break the number up, do the tens first, then bust up the rest of it. This will give you the exact answer quickly without ballparking it.

mea05key's avatar

24 times 12

4times 2 gives 8 (colum multiply)
cross multiply both numbers and add them 2times 2 + 4times 1 = 8
finally 2times 1 gives 2 (colum multiply)

thus 288

from speed math

squirbel's avatar

Base10 is my friend.

12+4 = 10 + 6 to me.
2347×7 = 2000×7 + 347

Like everyone here, I just break it down into simple chunks and then mush them back together.

stevenb's avatar

@squirbel, wouldn’t that be
2000×7 plus
300×7 plus
40×7 plus
Just checking LOL

augustlan's avatar

I am all about the 10s.

9×35 gets converted to
10×35 = 350 then
350 – 35 = 315

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