Mental Math Shortcuts

kalid · September 29, 2008, 9:56pm

Thanks, that’s my favorite trick of the bunch.

Anonymous_User · October 20, 2008, 8:38am

Thank you so much! Very helpful. I got a 98 on my quiz thanks to this.

kalid · October 20, 2008, 6:32pm

Awesome, glad it was helpful.

paulinediannemi · November 27, 2008, 12:18pm

nicE bLog,.,!

leilaniestrada · December 1, 2008, 9:56am

1)Write the 25th term in the number sequence: 2,3,5,7,11,13,17…(please explain using short-cut method). Thanks.
2)If 2+4+6+…+198+200= 10 100, what is the value of 1+3+5+…+197+199? (Solve using short-cut method)

Anonymous_User · December 11, 2008, 3:48pm

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000 · January 17, 2009, 3:38pm

What’s 43% of 200? Same as 200% of 43: 86

Can some people tell me how this work? To me its 43% of 200 which is 86, vs 200% of 43 which is 172, and 172:86 is not 86 but 2.
No matter how I do this I can’t get it to be
43% of 200 =86= 200% of 43: 86, which is basically what you are saying…

kalid · January 31, 2009, 11:47pm

@Anonymous: Hi there, 200% of 43 is

(200/100) * 43
2 * 43
86

Hope this helps.

Anonymous_User · February 6, 2009, 1:49pm

[…] More tricks and shortcuts Other great mental math tricks and shortcuts can be found at listverse.com and betterexplained.com. […]

Anonymous_User · February 15, 2009, 6:25pm

[…] Math short-cuts. […]

Anonymous_User · February 17, 2009, 7:22pm

[…] We’re like kids learning that 3 times 7 = 7 times 3. (Or that a% of b = b% of a — it’s true!). […]

Anonymous_User · March 6, 2009, 5:13am

Dear Kalid,Please take this into account and resolve the issue ASAP:c≈299,792.458 Km/s, not c≈300K Km/s.Calvin

Anonymous_User · March 6, 2009, 5:15am

…Why did my comment get butchered?

Anonymous_User · March 17, 2009, 3:04am

your formula for doubling time should be close to 70/X% the correct number is ln2x100/X%

Anonymous_User · March 17, 2009, 3:05am

your formula for doubling time should be closer to 70/X% the correct number is ln2×100/X%

kalid · March 17, 2009, 9:36am

@Calvin: Not sure I understand the difference; 300,000 Km/sec is a decent approximation.

@J: Yep, the “Rule of 70” is more accurate by 72 has more divisors so can be easier to work with (one reason it’s well known). I have another article on this.

Anonymous_User · May 7, 2009, 7:30am

[…] Intro: Mental math shortcuts, adding 1-100, how to learn math, understanding averages […]

vimalmistry · May 9, 2009, 8:16am

hi, i request u please all mathematical shortcut formula needs from u. for example 12 quare root 144 so shortcut formula is 1212=144
1 2
1 +2 1 + 1 = 22=4

1 4 4 1 is direct as second line
thank u

000 · June 8, 2009, 7:28pm

Hi,
I have a gd shortcut for multiplying 3 digit numbers:
109 x 105 = _ _ _ _ _
1 _ _ (add last 2 digits) then _ _ (multiply last two digits)
Answer 1 14 45 or 11445

Help me with other math shortcuts.

manishnigam · July 4, 2009, 9:18am

please give me shortcut tricks of pipes and cisterns