Mathematics Stack Exchange
Mathematics Stack Exchange

Questions tagged [mental-arithmetic]

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

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

115 questions
8
votes
3 answers

Mental Math: Finding Square Roots to 1 Decimal Point

I have 2 questions here. What is the most effective and easy way of calculating square roots in your head to an accuracy of 1 decimal point? This would need to work with at least two digit, non-perfect squares and would have to be doable…
m.smakg
  • 303
6
votes
1 answer

Soroban Abacus - how to memorise?

I saw a video where Indian kids where fiddling fingers in the air and working out big sums. WOW I thought. I researched loads and have figured out that it's all based on using the Soroban Abacus (a Japanese abacus if you don't know). I got a little…
Omar
  • 61
5
votes
2 answers

Fastest way to multiply numbers mentally?

I'm wondering what is the fastest way to multiply numbers? For now, let's focus on 2-digit numbers and were one cannot use scrap paper. I've come across 3 fast methods: 64x43 1) 60x43 + 4x43 (note that 60x43 is actually a 1-dig. times…
Cindy88
  • 221
4
votes
3 answers

Fastest way to multiply small numbers with decimals mentally

Is there a fast way to multiply these numbers mentally? Some examples: $0.85 \times 1.15$ $0.5 \times 1.5$ $0.2 \times 1.4$
Math is fun
  • 119
3
votes
4 answers

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 would have around 10 to 15 seconds for each…
M.Ross
  • 31
3
votes
5 answers

Is there an easy way to multiply two 2-digit numbers that have flipped digits?

I am currently a sophomore in high school competing in UIL academics in Texas. I am competing in the number sense test. Among the questions is the need to quickly multiply 2 two digits numbers, but as with all the questions present in the test,…
NoahGonzales
  • 31
2
votes
2 answers

How do you calculate bitwise XOR in your head?

Subtraction has an easy method: you can literally count down until you get it. So does addition, multiplication, division, etc. However, does bitwise XOR have a method like this that you can do in your head (apart from converting into binary)? If…
virchau13
  • 163
  • 6
2
votes
1 answer

Nice approximation to pattern

Does anyone know a good general approximation (within 5 percent) for the sum of the products of two numbers such that the sum of one number of one product and another of another product is equal two the other two numbers MENTALLY. Format:…
Ian L
  • 889
1
vote
1 answer

An app for learning the 1-100 times tables

Do anyone knows an app for android that help you learn $1$ to $100$ times table? I tried a few but none was quite what I need. I want it to: Ask answers for two digit by one digit multiplication. $23 \times 7$, $39 \times 3$ and so on. Ask based on…
Oriom Lyra Lisboa
  • 73
1
vote
3 answers

What is the method for mentally computing $3^{3.5}$ and similar calculations?

What is the method for mentally computing an approximation of $3^{3.5}$ and similar calculations? (without using any calculator) The best I did is: $3^{3.6}=e^{3.6ln(3)}=e^{3.6*1.098} \approx e^{3.6*1.1}=e^{3.96} \approx e^4=54.5$
astudentofmaths
  • 696
1
vote
1 answer

Proof behind this mental math multiplication?

Recently came across this technique of multiplying two $2$-digit numbers involving the same digit at the tens place and the sum of digits at units place being $10$. E.g., $73 X 77$ has same digit at the tenth place, viz., $7$ and the sum of digits…
user356774
1
vote
3 answers

Divison In Mental ArithMatic

The nearest number to $99548$ which is divisible by $687$ is? How can I find the answer quickly, is there any short cut to check if a number is divisible by $687$?
cloud computing in salesforce
  • 629
1
vote
3 answers

Is there a speedy mental algorithm for subtracting large numbers?

Like for example 65465-78954-12356 = -25845 Obviously the "borrowing" method that everyone learns in elementary school works, but it's slow and tedious, especially for results that come out negative.
nathanvy
  • 13
0
votes
2 answers

Understanding how This Works (shortcut for squaring $2$ digit numbers mentally)

We all know that math is as much about finding the answer as it is about knowing how a method leads you to the answer. In fact, not knowing the how has caused me to loose marks on several occasions at school. So my question arises, how does the…
m.smakg
  • 303
0
votes
1 answer

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 a value for n (3 in this case) and adjust m to so solve the…
Satvik Duddukuru
  • 1
1
2