Really Fun/Useful Math Hints for Fast Solving

[ReVolver]

Can you solve 103^2 in 3 seconds?

Have you tried doing this?

103 add 3 to 103

103+3= 106

3^2 = 9

10609 is the answer

Can you solve 221013/9 in 5 seconds?

Have you tried doing this?

221013

2

2+2 = 4

24

4+1 = 5

245

5+0 = 5

2455

5+1 = 6

24556

6+3 = 9

9/9 = 1

6+1 = 7

24557 is the answer

Can you solve 98x97 in 4 seconds?

98 => -2 (2 away from 100)
97 => -3 (3 away from 100)

98-3 = 95
97-2 = 95

95

-2(-3) = 6

9506 is the answer


That's all I got
Please share more

 

[darkRae]

98-2 = 95
97-3 = 95

Nice Math

The one with 9 is a bit strange though.
How would you do 99/9 ?

9 + 9 = 18 (?)

 

[ReVolver]

Nice Math

The one with 9 is a bit strange though.
How would you do 99/9 ?

9 + 9 = 18 (?)

Thanks for catching that (fixed)

-----------
9

9+9=18

18/9 = 2

9+2 = 11

 

[Insane!]

Im comming up on the CRCT! how would i do square roots in my head?
(i mess up alot)

 

[Bartuc08]

i don't get it... is it ways to solve the problems you already have? or is there a method that will solve all problems in short?

caus ei use weird shortcuts like this all the time, theres usually not a formula it mainly just luck and guessing

 

[Andyoyo]

Any number whos digits add up to a number of 3 example: 10000101
is divisible by 3. Same rule goes for 9.

If you ever multiplying by 5 just take half the number & multiply it by 10. I know that sounds obvious but no one does it
example: 88*5=?
88/2= 44
44*10=440
88*5=440

 

[Sil3nt]

For multiples of 11, learnt this at a tutor way back.

(A) (B) (C) (D)
11 x (1)[5] = (1) (1)+[5] [5] = 165
11 x (6)[2] = (6) (6)+[2] [2] = 682

If (B)+(C) > 10 then
(A) = (A) + 1 and (BC) = 2nd Digit (unit)
^For every 10 increase the addition of (A) by 1.

11 x (1)[9] = (1) (1)+[9] [9] = 209

(A) = (A) + 1 = 2 (BC) = 0 (10 <<2nd digit)

I'm not sure how 11 x 3 digits would work cause I never tried or bothered to figure it out.

 

[AgentPaper]

for multiplying by 11, you can just do this:

First number will be the first number you multiply by, then the second will be the first plus the second, then the second plus the third, and so on, until it's the second last plus plus the last, and finally the last by itself:

13542315233*11

148965467563

If the added number is 2 digits, you just add 1 to the digit to it's left, and use the second digit as the added number:

2760139*11

30361529

This makes it really easy to do small numbers in your head, for example 23*11=253, though larger numbers you might have to write it as you go, or keep numbers on your fingers.

This also works with 111, 1111, 11111, etc, though it's harder to do. basically, it goes, (for 111) 1st, 1st+2nd, 1st+2nd+3rd, 2nd+3rd+4th, 3rd+4th+5th, etc, until its the last three digits added, then the last two, and finally the last digit by itself. For more 1's, you basically add together at the same time however many 1's there are in the number. So, for 11111, you would add 5 at a time, with 10 1's, 10 digits at a time, and so on.


I use this all the time, though for anything more than 11 times a 3 digit number, a calculator is usually faster.

 

[Flare]

Can you solve 221013/9 in 5 seconds?

This is addressed to ReVolver mainly (and anyone who posted a hint for quick solving)

How long did it actually take you to think of these methods? I'll guess it was longer than 5 seconds It's handy if you get the particular problem to work out, but can't see it being fast otherwise.

Still cool, good thinking

 

[Trollvottel]

i always have my calculator with me if i want to calculate something. so i even type 2+9 in it. otherwise i would fail ^^.

But i found a way to multiplicate numbers with themselves:

for example 15*15:

5*5 = 25
5+5 = 10
10*10 = 100
so
100 = (5+5) * 10
025 = 5*5
100 = 10*10
=
225

same with 18*18


8*8 = 64
8+8 = 16
10*10 = 100

064
160
100
=
324

 

[diggoran]

Any number whos digits add up to a number of 3 example: 10000101
is divisible by 3. Same rule goes for 9.

If you ever multiplying by 5 just take half the number & multiply it by 10. I know that sounds obvious but no one does it
example: 88*5=?
88/2= 44
44*10=440
88*5=440

That's like my biggest math shortcut right there! All-day since fourth grade!

 

[Bartuc08]

i see what you did now!

lol
one that i always use is if its like X*9
so i'd do X*10-X

so if it's 4*9
then i'd do
4*10=40
40-4=36

 

[Flare]

haha, thats a nice way of doing it bartuc. add a 0 and take away the number, never wouldve thought of it

 

[duyen]

This is addressed to ReVolver mainly (and anyone who posted a hint for quick solving)

How long did it actually take you to think of these methods? I'll guess it was longer than 5 seconds It's handy if you get the particular problem to work out, but can't see it being fast otherwise.

Still cool, good thinking

I'm guessing he did not come up with these.

 

[Sim]

> Can you solve 103^2 in 3 seconds?

How come it doesn't work with 207^2?

I come up (with the calculator) with 42849, while your trick should give me 214049 or 21449 or something.

 

[SFilip]

> How come it doesn't work with 207^2?
Only seems to work for 101-109.

 

[Ninja_sheep]

The most common trick:
9*24

10*24 = 240
240-24 = 216

 

[Sintoras]

Just a hint, not really for fast solving.

the square of a number is always 1 larger than the loewr and higher numbers multiplied. The two below and above that are 3 less. The numbers three above and below are 9 less. This continues in a reglar pattern if I'm not mistaken.

Example:

5² = 25
4*6 = 24 = 1 less
3*7 = 21 = 4 less
2*8 = 16 = 9 less

 

[Flare]

3*7 = 21 = 3 less

Since when is 25-21 = 3

 

[Sintoras]

Sorry, typo