a, b and c are positive numbers. If a2b - c=2b3a + c=ab , what is the value of ab?

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[GMAT math practice question]

a, b and c are positive numbers. If a/2b - c=2b/3a + c=a/b , what is the value of a/b?

A. 1/3
B. 1/2
C. 2/3
D. 3/4
E. 4/5
Source: — Problem Solving |

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Remember that a/b=c/d=e/f=a + c + e/b + d + f.

When we add numerators and denominators, we have a/2b - c=2b/3a + c=a/b=a + 2b + a/2b - c + 3a + c + b= 2a + 2b/3a + 3b= 2(a + b)/3(a + b)=2/3.

Therefore, C is the answer.
Answer: C