1. I need to know if any of you can calculate .2 x 3.9 within 3 or 4 seconds using your mind only (no calculators or use of writing utensils) for the first time (as in you have not practiced this calculation or any particularly close it it before). I am doing some research and must know this. Please be honest!

Thanks.

2.

3. I'm a bit slow with mental math, but I can do the calculation farily easily (maybe 5-6 seconds). But this is a special case, since .2 is such a nice number to be multiplying by.

4. I could do it fairly quickly. Are you at all interested in the technique used to derive the answer?

5. 30 seconds for me. I guess one might say I am a bit slow, but I use the ephemism "careful."

6. Originally Posted by Cold Fusion
I need to know if any of you can calculate .2 x 3.9 within 3 or 4 seconds using your mind only (no calculators or use of writing utensils) for the first time (as in you have not practiced this calculation or any particularly close it it before). I am doing some research and must know this. Please be honest!

Thanks.
About 2 seconds, maybe a bit less. I have done thousands of similar mental calculations and do them all the time. That 2 seconds includes a quick check for accuracy.

7. I could do it fairly quickly. Are you at all interested in the technique used to derive the answer?
Yes, that is what I was getting to.

I am interested in the techniques that people use to acquire answers in various situations.

I watched a show on the Science Channel a few weeks ago that discussed (at least in part) methods that certain people use to calculate things at 'abnormal' speeds. Unfortunately they only went over a type of synthesis method (still extremely interesting though) and did not talk about more...conscious methods.

8. My logic was as follows, although the steps took varying amounts of time:
- 0.2 * 3.9
- 2/10 * 3.9
- (2 * 3.9) / 10
- (2*3 + 2*0.9) / 10
- (6 + 1.8) / 10
- 7.8 / 10
- 0.78

9. Personally I converted to and to , then multiplied to get . Looking at it as fractions, especially since the numerator in is a , makes it easy to multiply, rather than multiplying it in the forms and .

10. Originally Posted by MagiMaster
- (2 * 3.9) / 10
- (2*3 + 2*0.9) / 10
- (6 + 1.8) / 10
Why not

- (2 * 3.9) / 10
- (2*4 - 2*0.1) / 10
- (8 - 0.2) / 10

11. Originally Posted by Demen Tolden
30 seconds for me. I guess one might say I am a bit slow, but I use the ephemism "careful."
Method: 2 x 39 = 78 Now insert decimal point appropriately (folks who used to work slide rules learned how to figure out where the decimal point goes rather quickly(.

12. Originally Posted by DrRocket
Originally Posted by Demen Tolden
30 seconds for me. I guess one might say I am a bit slow, but I use the ephemism "careful."
Method: 2 x 39 = 78 Now insert decimal point appropriately (folks who used to work slide rules learned how to figure out where the decimal point goes rather quickly(.
That's the method I used. Seems like it took less than 4 seconds. Another slide rule veteran here.

13. Originally Posted by JaneBennet
Originally Posted by MagiMaster
- (2 * 3.9) / 10
- (2*3 + 2*0.9) / 10
- (6 + 1.8 ) / 10
Why not

- (2 * 3.9) / 10
- (2*4 - 2*0.1) / 10
- (8 - 0.2) / 10
I'm more used to taking things apart that way, basically. Also, I do better with addition.

14. That is what I did, 2 x 39=78 with the decimal moved over 2 places. If only they had told me that technique in school.....

It seems that I must teach myself math all over again and try to forget about what they "taught" me in school. The methods, or lack of methods that they showed me caused me to fear math, when really, it is terribly easy as long as you have something more intelligent than an insect teaching you it. Anyone here who went to school in a country other than America (or any lower on the list then us...only a couple though....but I cannot seem to think of any right now :? ) is very lucky. I watched a documentary a few years ago discussing math education in countries lower (towards 1) then us; it was completely different. Here, EVERYONE HATES HATES math; why? because the teachers are awful! And to add to that, the students are so rude and disrespectful that a proper teaching atmosphere can never be obtained! I grew up being told, "thats just how it works" from teachers. The only explanations I managed to find were in elementary school when after threatening my teacher with my failure (which at that school would have damaged her career) she was forced to take some of her dear time and actually show me how division and multiplication worked. It ended there; all math after that which was not based on pure memorization I taught myself.

15. Sorry for that tangent, I cannot help it sometimes :wink:

Going back to the techniques; If you look up synesthesia you might be able to find out more about mathematical techniques pertaining to it, though the easiest way would be to watch the science channel show on it. One women from the show apparently could turn colors into sets of numbers, displayed vividly in front of her; this allowed for instant calculations of large vegetable and fruit purchases. To prove that they were in fact experiencing these things, the scientists/psychologists on the show ran a few tests that were nearly irrefutable in their ability to prove that these abstract techniques were actually being used by these people.

16. For me, that would be maybe 10 seconds, I multiply 3.9 by 0.1 and add the results...

• 0.1 * 3.9 = 0.39
2 * 0.39 = 0.78

But that was an easy one, but overall I'm reallllllllllly slow with mental math! I guess it takes alot of practice!

17. Those bastards back in elementary school forced me to become reasonably good at mental math...which unfortunately hurt my grades in middle and high school-the morons declared I was cheating due to the fact that I did everything in my head.

18. I did it immediately upon reading the question - one, maybe two seconds.

The method was: double the 39, place the decimal by intuition about the size of the answer.

I am not quick at mental arithmetic in general, but I have taught math and arithmetic at various levels, including tutoring children who were just learning to handle the arithmetic of decimals and percentages, so have spent some time thinking about how best to handle such problems in different ways of thinking (different kids).

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