Mental Math - Percentages

[notsowisetooth]

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Does anyone have a good method for calculating percentages mentally?

I was taking a Kaplan Practice Test and I was spending too much time trying to calculate simple calculations.


For instance, how would i do 50/350 without having to go through all the steps of division writing down the radical, bringing down the 0, etc.?

😱

 

[iwannabadentist]

try to guesstimate. so if you have something like (totally random numbers) 587/673, don't do long division for that! try to round it up so its 590/675 which can be reduced to approx 24/27 which can be further reduced to 8/9 which you should know is slightly less than 90% just by looking at it - so you'd say like 88%
calculator answer for 587/673 is 87.2%!!
you can try to do that with any set of numbers see if it works! 🙂

 

[CCDDS]

50/350
=1/7
=~1/6
=~1/2*1/3
=~1/2*.3333
=~1/2*.3
=~.15%

 

[iwannabadentist]

50/350
=1/7
=~1/6
=~1/2*1/3
=~1/2*.3333
=~1/2*.3
=~.15%

it would be 0.15 but 15%. 👍

 

[CCDDS]

Yes. my bad.

 

[bbwreckerz04]

There's a chart of fractions that you should learn of what the decimals are it's from 1/2-1/10. Then you can reduce fractions and estimate from there.

For example 27/81:

(27/81)=(9x3)/(9x9),you then cancel the 9 and reduce to give 1/3 and you should know 1/3 equals .333

 

[Streetwolf]

50/350
=1/7
=~1/6
=~1/2*1/3
=~1/2*.3333
=~1/2*.3
=~.15%

Waaaaay too much estimation. You're lucky the change from .333 to .3 balanced out the 1/7 becoming 1/6.

OP, you should know that 1/7 is about 0.1428 (so 14.28% here).

Otherwise you need to round the numbers REASONABLY. You won't get anything ridiculous on the DAT. A good first step would be to eliminate obviously wrong answers. If you had 674/1203 then you could definitely eliminate any answer under 50% (about 600 numerator) and safely eliminate anything over 67% (about 800 numerator). It would be somewhere NEAR the middle of those two but slightly toward 50%. I'd guess around 56%. Turns out it's 56.03% so that's damn close.

 

[notsowisetooth]

Hey guys,

Another quick question.

What's the best way to estimate square roots of small numbers like
8.5 x 10^-17?

My first instinct is to find a number of the order of around 10^-8, but I have no idea how the first part (8.5) affects the order of the number.

help!

 

[mmorris86]

Hey guys,

Another quick question.

What's the best way to estimate square roots of small numbers like
8.5 x 10^-17?

My first instinct is to find a number of the order of around 10^-8, but I have no idea how the first part (8.5) affects the order of the number.

help!

What you would do here is change it to 85 x 10^-18 just by moving the decimal place over one, then you take the square root of each number separately:

sqrt (85) ~ 9

sqrt (10^-18) = 10^-9 (this is a good rule for exponents to memorize, when u take the square root of an exponent you halve it)

so its around 9 x 10^-9.... its actually 9.2195 x 10^-9

Close enough 👍

 

[UCB05]

50/350= 150/1050 which almost equals 150/100 or 15%

 

[mmorris86]

There's a chart of fractions that you should learn of what the decimals are it's from 1/2-1/10. Then you can reduce fractions and estimate from there.

For example 27/81:

(27/81)=(9x3)/(9x9),you then cancel the 9 and reduce to give 1/3 and you should know 1/3 equals .333

The Numbers---How to memorize them
1/2 = .5 = 50%---Easy

1/3 = .33 = 33%---Easy

1/4 = .25 = 25%---One quarter = 25 cents

1/5 = .2 = 20%---5 $20 bills gives you a $100

1/6 = .167 = 16.7%---You add 1+6=7

1/7 = .143 = 14.3%---1 in front is kinda obvious at this pt 4+3=7

1/8 = .125 = 12.5%---1+2+5=8

1/9 = .111 = 11.1%---10-9=1 over and over again

1/10 = .1 = 10%---Easy

Sorry if the above is gay, I'm really tired, can't sleep or study anymore, and I'm not hungry.. what else is there to do!?!

 

[jay47]

The Numbers---How to memorize them
1/2 = .5 = 50%---Easy

1/3 = .33 = 33%---Easy

1/4 = .25 = 25%---One quarter = 25 cents

1/5 = .2 = 20%---5 $20 bills gives you a $100

1/6 = .167 = 16.7%---You add 1+6=7

1/7 = .143 = 14.3%---1 in front is kinda obvious at this pt 4+3=7

1/8 = .125 = 12.5%---1+2+5=8

1/9 = .111 = 11.1%---10-9=1 over and over again

1/10 = .1 = 10%---Easy

Sorry if the above is gay, I'm really tired, can't sleep or study anymore, and I'm not hungry.. what else is there to do!?!

Great method, I've never seen this before! Definitely memorize ALL of these values, they will surely help you on the real DAT.

As far as estimating, the best way to do things is to always bring it back to a whole number.

For example, what is 15% of 489? Seems kinda tough, but it can be done all in your head in about 10 seconds.

Okay, let's bring it back to a number we can work with: 10%
What is 10%=48.9
Then, just find out what half of 48.9 is because that is going to be 5%.

So you are breaking a more complex problem down into simpler parts.\

10%=48.9
5%= 24.45

=73.35

I did that all in my head and I'm not even that good at math!