Questions tagged [mental-arithmetic]
Mental arithmetic comprises arithmetical calculations using only the human brain, with no help from calculators, computers, or pen and paper.
119 questions
12 votes
2 answers
1k views
I found a way to multiply numbers, is this a new method or someone else had already found this?
I recently noticed a fast way to multiply two numbers near $50$. Let the numbers be $a$ and $b$ both close to $50$. Define $$x = 50−a,\ y =50−b$$ (so $x,y$ are the “deficiencies” from $50$ ) My ...
- 129
6 votes
2 answers
508 views
How to determine , by mental arithmetic, the cube root of 188,132,517 is 573? Question is from Hua LooKeng.
From an article by Chinese mathematician LooKeng-Hua: "Someone can blurt out that the cube root of 188,132,517 is 573. Of course, 188 determines the first digit 5, and the last digit 7 determines ...
- 69
1 vote
1 answer
62 views
Expected Length of Walk on Truncated Icosahedron
Consider a truncated icosahedron with 12 pentagons and 20 hexagons. Starting from a hexagonal face, we go to any neighboring polygon randomly with equal probability. What is the expected number of ...
- 372
0 votes
1 answer
292 views
Does practicing mental math increase mathematical intuition? [closed]
Has any experienced mathematician on this site found that practicing mental math was associated with the development of mathematical intuition and conceptual understanding? I was considering ...
- 3,152
1 vote
2 answers
641 views
Trick for calculating the 5th order derivative of $(2x^3+1)e^{x^2}$ evaluated at $0$
Are there any tricks for calculating the 5th order derivative of $(2x^3+1)e^{x^2}$ evaluated at $0$? I guess it involves binomial expansion but it still seems too complicated.
- 13
1 vote
0 answers
79 views
How to quickly eyeball maximum sum subarray?
Given an integer array, I want to find the continuous subarray (containing at least one number) which has the largest sum. There are some algorithmic solutions to this here: https://en.wikipedia.org/...
- 1,446
0 votes
0 answers
6k views
In time t, a dog does 3 jumps whereas a fox does 5. If distance covered in a jump by a dog is thrice that of the fox, find ratio of speed of dog : fox
This looks (and I think it is) a pretty simple problem. My reasoning is as follows : ...
- 361
1 vote
4 answers
620 views
Is there a reasonably accurate and easy way to approximate lbs-stones or kg-stones (and vice versa)?
For example, when I need to talk with my American friends about body weight: $\mathrm{KG}\times2+10\%$ ($100\times2=200$, $200+10\%=220$... Pretty accurate and easy to do mentally, both ways) However, ...
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