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How to do some calculation fast

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by [email protected] » Sat Apr 06, 2013 11:58 am
Hi,
Please suggest some quicker way to solve practice questions Q#135 and #182 of Problem Solving in OG 13th Edition.

As per the explanation of Q.135, they have done calculations:
207/1.15=180 how can we do this calculation fast.

In Q.182 explanation also some lengthy calculations have been done, which takes time.

Please suggest some tips for such questions.

Thanks!
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by rintoo22 » Sat Apr 06, 2013 12:13 pm
Hi Depali,

Please post the questions.
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by srcc25anu » Sat Apr 06, 2013 1:17 pm
1.15 * 2 = 2.30
so 2.07 * 100 / 1.15 = (Slightly less than 2) * 100
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by GMATGuruNY » Sat Apr 06, 2013 3:50 pm
PS182 in the OG13:
A photography dealer ordered 60 Model X cameras to be sold for $250 each, which represents a 20 percent markup over the dealer's initial cost for each camera. Of the cameras ordered, 6 were never sold and were returned to the manufacturer for a refund of 50 percent of the dealer's initial cost. What was the dealer's approximate profit or loss as a percent of the dealer's initial cost for the 60 cameras?
A.7% loss
B.13% loss
C.7% profit
D.13% profit
E.15% profit
54 cameras yield a 20% profit, while 6 cameras suffer a 50% loss.
Average for all 60 cameras = (54*20 - 6*50)/60 = 13.

The correct answer is D.
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by GMATGuruNY » Sat Apr 06, 2013 3:54 pm
PS135 in the OG13:
The price of lunch for 15 people was $207.00,
including a 15 percent gratuity for service. What was
the average price per person, EXCLUDING the
gratuity?

(A) $11.73
(B) $12.00
(C) $13.80
(D) $14.00
(E) $15.87
We can plug in the answers, which represent the cost per person.
Given that the total cost is an INTEGER, the correct answer is almost certainly an integer itself.
Answer choice D is too great, since 15 people paying $14 each = 15*14 = 210 -- a value beyond the total cost of 207.

Answer choice B: $12 per person without gratuity
Total price for 15 people = 15*12 = 180.
Price + gratuity = 180 + .15(180) = 180+27 = 207.
Success!

The correct answer is B.
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by Anju@Gurome » Sat Apr 06, 2013 7:05 pm
[email protected] wrote:207/1.15=180 how can we do this calculation fast.
Prime factorization.
207 = 3*69 = 3*3*23
115 = 5*23

So, 207/1.15 = 207*100/115 = 9*100/5 = 180
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Backup Methods : General guide on plugging, estimation etc.
Wavy Curve Method : Solving complex inequalities in a matter of seconds.

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by rintoo22 » Sun Apr 07, 2013 1:08 am
The price of lunch for 15 people was $207.00,
including a 15 percent gratuity for service. What was
the average price per person, EXCLUDING the
gratuity?

(A) $11.73
(B) $12.00
(C) $13.80
(D) $14.00
(E) $15.87
x+15/100(x)= 207
x=180
180/15 = 12 ... Ans B.
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by bharat.bondalapati » Sun Apr 07, 2013 4:54 am
For fast calculations, learn squares and cubes upto 30, multiplication tables atleast upto 20 and also learn to quickly factorize larger numbers. Hope that helps.
Best,
Bharat

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