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fractions

by gmatmachoman » Sun May 30, 2010 9:47 am
The fraction 2/7 can be written in a unique way as the sum of two unit fractions 1/a + 1/b, where a and b are positive integers with a < b. Then a + b is

(1) 16 (2) 20 (3) 24 (4) 28 (5) 32


** Guys, is this a GMAT type question?
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by kaulnikhil » Sun May 30, 2010 10:39 am
gmatmachoman wrote:The fraction 2/7 can be written in a unique way as the sum of two unit fractions 1/a + 1/b, where a and b are positive integers with a < b. Then a + b is

(1) 16 (2) 20 (3) 24 (4) 28 (5) 32


** Guys, is this a GMAT type question?
Is it D 28?
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by thephoenix » Sun May 30, 2010 10:42 am
IMO E

a+b=2ab/7
1) a+b=16--->ab=56 --->ab=2*2*2*7 with all combination we will not get a+b as 16
2) a+b=20 --->ab=70=2*5*7 [possible combination are 2,35;10,7;14,5]
3)24--->ab=84=2*2*3*7 not possible
4)28--->ab=98=2*7*7
5)32--->ab=16*7=2*2*2*2*7=4*28---->yes 4+28=32 and 4*28=112 correct
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by clock60 » Sun May 30, 2010 11:06 am
also got E
1/28+1/4=2/7
with b=28, and a=4
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by selango » Sun May 30, 2010 8:06 pm
what is the source of this question?
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by gmatmachoman » Sun May 30, 2010 10:32 pm
thephoenix wrote:IMO E

a+b=2ab/7
1) a+b=16--->ab=56 --->ab=2*2*2*7 with all combination we will not get a+b as 16
2) a+b=20 --->ab=70=2*5*7 [possible combination are 2,35;10,7;14,5]
3)24--->ab=84=2*2*3*7 not possible
4)28--->ab=98=2*7*7
5)32--->ab=16*7=2*2*2*2*7=4*28---->yes 4+28=32 and 4*28=112 correct
Bhai, how do u sense the degree of difficulty of this one???in terms of time taken!
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