# Questions tagged [mental-arithmetic]

Mental arithmetic comprises arithmetical calculations using only the human brain, with no help from calculators, computers, or pen and paper.

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### 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 ...
1 vote
145 views

### Can you propose any hack for remembering multiplication tables from $13-19$?

I memorized multiplication tables from $2-12$ when I was in elementary school. Regarding tables $13-19$: I remember some intermittent elements I find others by multiplication by recombination. E.g., ...
1 vote
391 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.
35 views

### Mental math to quickly solve a product of mixed numbers with variables

I've come across the following question: "Let $6\frac{1}{m}\times n\frac{2}{11}=21$, where m, n are natural numbers. Find m+n." Outside of actually solving the question, I've tried to guess ...
1 vote
71 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 vote
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1 vote
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### Prove that $e^\pi > 21$. Without a proof, you can use the two facts: $e > 2.71$ and $\pi > 3.14$.

I heard that the following problem is for high school students. Prove that $e^\pi > 21$. Without a proof, you can use the two facts: $e > 2.71$ and $\pi > 3.14$. My solution is the ...
500 views

### How to solve a fraction with a numerator in exponential form and a denominator in numerical form without a calculator?

The question: "Imagine unwinding (straightening out) all of the DNA from a single typical cell and laying it "end-to-end"; then the sum total length will be approximately $2$ meters. ...
290 views

### Multiplication of decimal numbers

I was wondering if anyone knows any good resources to use or tricks to be able to solve these kinds of mental-arithmetic questions(See image below)? Would be really grateful for any help ! Edit: I ...
1k views

### Mental arithmetic: Add numbers by combining numbers that add up to 10

I'm reading a book on mental arithmetic, and came across this paragraph: You were taught - or should have been taught - at school that speed in addition is acquired by combining pairs of successive ...
84 views

### I want to learn math do I need to memorise the times tables?

As a front end developer, I do not need Math, however, I want to seriously learn it so that, maybe, one day, I might be able to get so good at it as to get into AI. I started with Algebra 1 and ...
119 views

### Whats best method to calculate multiplication mentally?

I would like to know which method is better for mentally multiplication calculation between a) Cross-Multiplication and b) Left-to-Right Multiplication Thank you!
344 views

2k views

### Mixture and Alligation:::

A solution contains alcohol and water in the ratio 3:1, 16 litres of the Mixture is drawn off and 12 litres of water is added.11 litres of Mixture is replaced by 11 litres of water.The alcohol and ...