math tricks thread

[savvysearch]

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Does anyone know any shortcuts for quantitative reasoning? This is my most daunting section and I need to know some shortcuts to get through this section as fast as possbile, so Im starting this thread for everyone to post some shortcuts in math.

I'll go first:

how to compare fractions to see which is the largest:

1/5 3/7 4/8 5/9

notice for each fraction, the denominator is 4 more than the numerator for all fractions. If this is the case, the larger number is the largest fraction, 5/9.

find the average of 8,9,10,6,7.

notice you can rearrange the numbers to 6,7,8,9,10 and they are consecutive, so the average is the middle number 8.

 

[yosemitesam]

Well, I started studying for the DAT after two semesters of physics and calculus. I had to have my TI-86 surgically removed from my hand... and after the surgery, I found that I couldn't even figure a 20% tip on a $10 meal without a calculator. Seriously, I couldn't function without that thing. And needless to say, I totally bombed the Quant. Thankfully my other scores were good. Anyway, my point is... well... ummm... good luck. 🙂

Also, don't get freaked out by TOPScore or Kaplan. They are way overpreparing you for the caliber of questions you will see on the test.

 

[tinker bell]

Most of the math on DAT, you can just guess.
If it takes you more than 30 seconds for a problem, you are probably on the wrong track.

For the above problem:
how to compare fractions to see which is the largest:

1/5 3/7 4/8 5/9

the largest number cannot be 1/5 and 4/8

Compare between 3/7 and 5/9. it's safe to guess that 5/9 is larger
And this only takes 5 seconds.

Tinker Bell-TEMPLE DENTAL 007😎

 

[savvysearch]

Oh, I totally remember that method of comparing fractions to 1/2. Thanks a lot!

By the way, I really like that advice of:

9/12, 5/10. Compare by multiplying 9 times 10 and 5 times 12. Since 9 times 10 is larger, 9/12 is larger.

I saw that advice here somewhere and it was really useful.


This one is bizzare:

252/350 = X/ 100.

One way is to multipy 252 by 100 to get 25200 and divide that by 350. Very long and tedious.

Another way, (that I read from some book) is to reduce 252/350 to 18/25. So 18/25=X/100. Since 25 times 4 is 100. 4 times 18 is 72 so X is 72. This is supposed to be easier but I still find it time consuming.

How do you know that 252/350 can be reduced by a multiple of 14? That the time consuming part. I just can't see that off the top of my head. Does anyone know a faster method? Or is there a trick to know what multiples fractions can be reduced by?

 

[groundhog]

Savvysearch,
You had to be one of the lucky (?) few who were required to learn their times tables up through the 99's in primary school rather than just through the 12's like most of us.....Quick.. off the top of your head.... what is 73 X 47?

 

[savvysearch]

But that's exactly it, groundhog. How am I supposed to know to reduce that fraction by 14? My arithmatic book says to do that, but that's impossible!