https://www.beatthegmat.com/tough-algebr ... 13568.html
Awesome explanation from Ian.
My approach if we dint recongnize this. For a word prob if the answer choices are not in terms of variables but actual numbers then reverse engineering would be a good approach.
Given : x<y
Start from C
k=15
k=10x+20y/x+y
10x+20y = 15x+15y
5y=5x
y=x
Cant be true since we are given x<y so k must be greater than 15
k=18
10x+20y = 18x+18y
2y = 8x
y=4x
Possible
k=30
10x+20y = 30x+30y
-10y = 20x
y = -2x
Not possible since x and y are both positive and x<y
Choose D)
P.S: Ian's solution would be the easiest and the best but instead of randomly guessing(if we dint recognize this as a weighted average problem) the reverse engineering could serve as a back up method for solving the problem. In GMAT we dont have to go with a text book approach if somethign else strikes us first i.e as long as we can get to the answer within 2 minutes.
Just my 2 cents.... Good luck!
Regards,
CR
GmatFoc
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Source: Beat The GMAT — Problem Solving |
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cramya
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Sometimes TacoBell's "Think outside the box" helps in tests like GMAT. Lets use it with caution though.... 
If u had the time lets check D
y=4x
x=1 y=4
10(1) + 20(4) / 1+4 = 18
10+80 / 5 = 18
90 = 90
Bingo... Move on.....
Regards,
CR
If u had the time lets check D
y=4x
x=1 y=4
10(1) + 20(4) / 1+4 = 18
10+80 / 5 = 18
90 = 90
Bingo... Move on.....
Regards,
CR
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logitech
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- GMAT Score:730
This equation can be condensed to:
10(x+y)/(x+y) + 10(y/(x+y))
We know the first part of the equation is 10 - so actually the problem asks what y/(x+y) can be...and we have a tip x<y
if x=y , our equation is 0.5
so we know that the answer needs to be more than 15
If x=0 , our equation gives us = 1 so the answer needs to be less than 20
15 < ANSWER < 20
And only 18 fits the bill
10(x+y)/(x+y) + 10(y/(x+y))
We know the first part of the equation is 10 - so actually the problem asks what y/(x+y) can be...and we have a tip x<y
if x=y , our equation is 0.5
so we know that the answer needs to be more than 15
If x=0 , our equation gives us = 1 so the answer needs to be less than 20
15 < ANSWER < 20
And only 18 fits the bill
LGTCH
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