If 4a = 7b = 21c, what is the value of b?

Question

If 4a = 7b = 21c, what is the value of b?
 
(1) a = 7/4

(2) c = 1/3

GMATinsight · Answer

Let, 4a = 7b = 21c = 84x

i.e. a = 21x
i.e. b = 12x
i.e. c = 4x

Question: b = 12x = ?

Statement 1 : a = 7/4

i.e. a = 21x = 7/4 i.e. x = 1/12
so b = 12*(1/12)

SUFFICIENT

STatement 2 : c = 1/3

i.e. c = 4x = 1/3 i.e. x = 1/12
so b = 12*(1/12)

SUFFICIENT

Answer: Option D

NitishJain · Answer

IMO:  D

QTN: 4a = 7b = 21c; find b.
4a = 7b ==> b= (4/7)a ----1
7b = 21c ==> b= (21/7)c or b = 3c. ----2

Statement 1: a = 7/4; Putting this in eq 1; b =1  SUFFICIENT 
Statement 2: c = 1/3; Putting this in eq 2; b =1  SUFFICIENT

chetan2u · Answer

If 4a = 7b = 21c, what is the value of b?

All three are related to each other, so value of any one variable will give all three values.  
 
(1) a = 7/4
a is given, we can find b and c.
Suff

(2) c = 1/3
c is given, we can find b and a.
Suff

D
 Never waste time on a DS question, if you can be sure that the given info is sufficient to get an answer.

Archit3110 · Answer

given
4a=7b=21c
so to get value of b we need either value of a or c
#1
a = 7/4
c = 1
sufficient
#2
c = 1/3
b=1
sufficient
OPTION D

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