the multiplication result of (x11) should be 108647.
anon
30-Apr-2011
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= (4 x 7) x (10^9)/(12 x 10^6) = 48 x 1000/8 = 6000
How does (4 x 7) x (10^9)/(12 x 10^6) equal 6000?
My calculation gets me 2333.33
anon
04-Mar-2011
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Hi, I have noticed there is a typo in the section about multiplying with 11. In the second example with carry over it reads 9877 *11 but should read (as far as I can see) 9867 as is in the rest of the example.
anon
07-Dec-2010
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A lot of ratio / percentage questions can be solved quite quickly if you remember these ratios. I have found them quite helpful.
1/2 = 50%
1/3 = 33%
1/4 = 25%
1/5 = 20%
1/6 = 16%
1/7 = 14%
1/8 = 12.5%
1/9 = 11.1%
1/11 = 9%
1/12 = 8.3%
1/13 = 7.7%
1/14 = 7%
1/15 = 6.7%
brad11
16-Nov-2010
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A quick way to finding how long it takes for a quantity to double: Rule of 72 or 69
It's usually good to know if X changes at Y% how long it takes to become 2X
Here are 3 ways
(a) Remember common %
5% 14.2
10% 7.3
20% 3.8
(b) Rule of 72. Simply do 72 / (Rate) and the answer, for lower rates, is a good enough approximate
i.e. 72 / y
5% : (72 / 5) = 14.4
10% : (72 / 10) = 7.2
20% : (72 / 20) = 3.6
(c) Rule of 69
Same as rule of 72 but 69 gives a higher level of accuracy. If you use the equation
2 = (1 + r)^t and solve for t, you would get ~0.69/r
Curious? you can read more about this on Wikipedia - http://en.wikipedia.org/wiki/Rule_of_72
admin
03-Nov-2010
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tip: Multiplying by 11
(search the web for the Trachtenberg System of Mathematics for more nuggets)
write the number with a leading zero - e.g. 4323 = 04323
then add each digit to the digit on the right
0 4 3 2 3
= 4 7 5 5 3
If you have a carry over, add the carry-over to the next left operation
e.g. 9877 x 11 = 09867 x 11
= (0 + 9 + 1) (9+8 + 1) (8+6 + 1) (6+7) (7)
= 10 8 5 3 7 = 108537
do it a couple of times and x 11 of any number will be a breeze.
admin
27-Oct-2010
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tip: Handling zeros
problem: if a question has a lot of zeros, it gets confusing very quickly. One good way to manage it without getting confused is to use the 10^x (10 to the power of) notation
100,000 = 10^5
1 million = 10^6
1 billion = 10^9
so, a simple example, for e.g. 400000*120000/8000000
= (4 x 10^5) * (12 * 10^4)/(8*10^6)
= (4 x 7) x (10^9)/(12 x 10^6) = 48 x 1000/8 = 6000
This can be very handy in verbal interviews (like case interviews) where you have to do calculations without a calculator and with the interviewers eyes on you and your paperwork.
admin
27-Oct-2010
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