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.

107 questions
0
votes
2 answers

Multiplication tables: up to what number should I memorise?

I never memorised them and suffered immensely through school as a result. Eg. I calculated 7x8 as (7x10=70) minus 7 using my fingers (63) then minus again on my fingers (56). In Australia children are encouraged to memorise multiplication up to 12…
0
votes
4 answers

Without using a calculator how to solve $x^x = 100$?

Without using a calculator how to solve $x^x = 100$ ? A way of finding an approximation to 2 decimals would be good neough. I know about the Lmabert W function but one cannot compute it mentally. This is why I believe my question not to be a…
astudentofmaths
  • 666
  • 5
  • 19
0
votes
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
0
votes
1 answer

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 water ratio is 9:13.Find the Intial amount of…
0
votes
2 answers

Multiplying very small numbers without the use of a calculator

As part of my college course I have to sit some math exams and I cannot use a calculator. The problem is I suck at maths with a calculator not to mind how bad I am at it without using a calculator! Anyways I'm wondering if anyone has any tips for…
Paul
  • 1
  • 1
0
votes
3 answers

Mentally calculate the first 9 terms of this Fibonacci sequence.

There was a question in a mental math test and it expected me to calculate the first 9 terms of the following Fibonacci sequence (Note that this is how the problem was exactly given.): The sum of the first nine terms of the Fibonacci sequence…
Ian L
  • 785
  • 4
  • 23
0
votes
4 answers

Problems on Trains

A train Leaves station $x$ at $5 AM$ and reaches station $y$ at $9AM$. Another train leaves station $y$ at $7AM$ and reaches station $x$ at $10.30 AM$ . At what time do the two trains cross each other? I have tried: Let the trains cover the (same)…
0
votes
2 answers

Mental maths for - $ \begin{pmatrix} 9\\ 4 \\ \end{pmatrix}$, $6^5$

How can I calculate the following things in my head? $ \begin{pmatrix} 9\\ 4 \\ \end{pmatrix} $ I know this is $\frac{9\cdot 8\cdot 7\cdot 6}{4\cdot 3\cdot 2\cdot 1}$ and then $3\cdot 2\cdot 3\cdot 7$ but I can't immediately come up…
z_z
  • 557
  • 1
  • 5
  • 6
0
votes
2 answers

Simplifying this fraction in a different base

Note: I would appreciate a solution that DOES NOT convert back to base 10. How would one simplify $\frac{43}{70}_8$? I assume, like in decimal, I must recognize a common factor and divide by that factor. Keyword is recognize because we are taught…
Ian L
  • 785
  • 4
  • 23
0
votes
1 answer

Is there a simple way of computing when $a^n=b^m$

I don't want exact equality just close enough to be useful in approximation. i.e. $2^{10 }= 10^3$ is very useful and used daily for an approximation. Is there a do this efficiently? Is there a way to do this in my head generally?
0
votes
0 answers

How to develop number sense while finding out the middle/median value?

I have seen that many people can find out the middle value between numbers orally. My question is how to find the middle value orally? For example , the middle value between - 24576.5 MHz — 24881.5 MHz Please don't tell me $\frac{24576.5 +…
dexterous
  • 591
  • 4
  • 10
  • 20
-1
votes
1 answer

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!
-1
votes
2 answers

Age questions: How to approach solving this?

Completely foxed by this question: Moses is twice as old as Methusaleh was when Methusaleh was one-third as old as Moses will be when Moses is as old as Methuselah is now. The difference in their ages is 666 years. How old is Methusaleh now? My…
-1
votes
1 answer

Boats and Streams

A boat can travel with a speed of $13$ km/hr in still water. If the speed of the stream is $4$ km/hr, find the time taken by the boat to go $68$ km downstream. Can you Explain the Difference between speed and Relative Speed? Share some basic…
-2
votes
1 answer

Can a mental calculator plot any function in seconds?

As we learnt at school, to draw the graph of a function in details it may be necessary to perform some steps of calculation such as finding domain and codomain, limits, asymptotes and sign of derivatives. These steps may require minutes of calculus…
sound wave
  • 795
  • 1
  • 6
  • 14