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# Using a Smart Number (Technique for PS)

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Manager
Joined: 14 Mar 2011
Posts: 50
Using a Smart Number (Technique for PS)  [#permalink]

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01 Sep 2011, 05:50
4
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I found this technique on Manhattan Series. I would like share my learning with you.

Sometimes, fraction problems on the GMAT include unspecified numerical amounts;
often these unspecified amounts are described by variables. In these cases, pick real numbers
to stand in for the variables. To make the computation easier, choose Smart Numbers equal
to common multiples of the denominators of the fractions in the problem.

Example

Lisa spends 3/8 of her monthly paycheck on rent and 5/12 on food. Her roommate, Carrie, who earns twice as much as Lisa, spends 1/4 of her monthly paycheck on rent and 1/2 on food. If the two women decide to donate the remainder of their money to charity each month, what fraction of their combined monthly income will they
donate?

Use Smart Numbers to solve this problem. Since the denominators in the problem are 8, 12, 4 & 2, assign Lisa a monthly paycheck of \$24. Assign her roommate, who earns twice as much, a monthly paycheck of \$48. The two women's monthly expenses break down as follows:
Rent Food Leftover
Lisa 3/8 of 24 + 5/12 of 24 = 9+10 Leftover = 24-19 = 5

Carrie monthly paycheck = 48

Carrie 1/4 of 48 = 12, 1/2 of 48 = 24---Leftover= 48 - (12 + 24) = 12

The women will donate a total of \$17, out of their combined monthly income of \$72.

Intern
Joined: 19 May 2014
Posts: 30
Re: Using a Smart Number (Technique for PS)  [#permalink]

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21 May 2014, 00:19
1
This is a great time-saving technique.

Here's another question to practice this technique:

Sarah and Joe are a couple. Sarah puts 1/9 of her monthly savings in Fixed Deposits, 5/12 in Mutual Funds and the rest in their joint savings account. Joe, who saves 40% more than Sarah, puts 1/8 of his savings in a Fixed Deposit, 5/6 in Mutual Funds and the rest in the savings account. What fraction of their combined savings lies in the joint savings account?

SOLUTION

In the post above, Carrie's salary was a whole number multiple of Lisa's salary. (Carrie's salary = 2*Lisa's salary). In the current question however, Joe's savings are a fractional multiple of Sarah's savings:

Let Joe's savings be J
Sarah's savings be S

J = (1+40/100)*S = (1+2/5)*S = 7/5*S

So, when we apply the smart number technique, we should also consider the fraction 7/5 while choosing our number.

How do we choose a smart number for Sarah's savings?

The fractions in play here are: 1/9, 5/12, 7/5, 1/8, 5/6

So, we will choose the Least Common Multiple of 9, 12, 5, 8,6.

That number will be 360.

So, S= 360

So, Sarah's Fixed Deposit, SF= 360/9 = 40
Sarah's Mutual Funds, SM= (5/12)*360 = 150
Sarah's Saving Account Deposits, SS= 360 - (40+150) = 170

J = (7/5)*360 = 504

Joe's Fixed Deposit, JF= 504/8 = 63
Joe's Mutual Funds, JM = (5/6)*504 = 5*84 = 420
Joe's Saving Account Deposits, JS = 504 - (63+420) = 21

So, Total Fraction in Savings Account for the couple = (SS+JS)/(S+J) = (170+21)/(360+504) = 191/864
Intern
Joined: 20 May 2014
Posts: 34
Location: India
Schools: IIMC
GMAT 1: 700 Q51 V32
Re: Using a Smart Number (Technique for PS)  [#permalink]

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22 May 2014, 06:53
1
Caution while using Smart Number !

Apologize for playing the devil's advocate

Never use Smart number if question involves values (not ratio or %ages) or Final question to be answered requires a value. Example, if we need to find out the total donation (anyways more info. would be provided to answer this).
Manager
Joined: 11 Jun 2015
Posts: 80
Location: India
Schools: CMU MSPM "21 (A)
Re: Using a Smart Number (Technique for PS)  [#permalink]

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02 Apr 2018, 06:16
Can anyone explain the answer ti this questions ?

Sarah and Joe are a couple. Sarah puts 1/9 of her monthly savings in Fixed Deposits, 5/12 in Mutual Funds and the rest in their joint savings account. Joe, who saves 40% more than Sarah, puts 1/8 of his savings in a Fixed Deposit, 5/6 in Mutual Funds and the rest in the savings account. What fraction of their combined savings lies in the joint savings account?
Manager
Joined: 17 Jan 2017
Posts: 59
Re: Using a Smart Number (Technique for PS)  [#permalink]

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02 Apr 2018, 10:30
Hi renjana,

is there a specific part of the explanation you don't understand? I would not waste time on calculating the exact amounts for fixed dep and mutual funds. You just need to calculate what part of the savings goes into the savings account.

Let Joe's savings be J
Sarah's savings be S

J = 1.4*S = 14/10*S = 7/5*S

So, when we apply the smart number technique, we should also consider the fraction 7/5 while choosing our number.

How do we choose a smart number for Sarah's savings?

The fractions in play here are: 1/9, 5/12, 7/5, 1/8, 5/6

So, we will choose the Least Common Multiple of 9, 12, 5, 8 and 6, since we want to deal with integers (not fractions).
9=3*3
12=2*2*3
5=5
8=2*2*2
6=3*2

That number will be 360 (2*2*2*3*3*5).

So, S= 360
How much of 360 is left after the reduction (fixed dep + mutual funds)?
4/36 of S -> Fixed
15/36 of S -> Mutual fund
--------------------------
36/36 - 19/36 = 17/36 (Sarah puts a little less than half of her savings in the savings account)
17/36*360=170

J = (7/5)*360 = 504

How much of 504 is left after the reduction (fixed dep + mutual funds)?
3/24 of J -> Fixed
20/24 of J -> Mutual fund
-------------------------
24/24 - 23/24 = 1/24 (Joe puts 1/24 of his savings in the savings account)
1/24*504=21

Together, they put (170+21)/(360+504) = 191/864 in the savings account
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Joined: 09 Sep 2013
Posts: 13733
Re: Using a Smart Number (Technique for PS)  [#permalink]

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09 Dec 2019, 15:17
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Re: Using a Smart Number (Technique for PS)   [#permalink] 09 Dec 2019, 15:17
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