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GMATWhizard

it says a>b, how are we taking a and b to have the same value then ?
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GMATWhizard

it says a>b, how are we taking a and b to have the same value then ?


Hey,


Thanks for pointing this out. If a > b, then we cannot consider the same value for a and b. I have edited the solution.


There is an alternate method that doesn't require us to take values and may help in avoiding such mistakes. (I am just showing the third step)

Step 3: Analyse Statements by combining.


From statement 1: a – b = n, where n is an integer .....(i)
From statement 2: a + 2b =m, where m is an integer.......(ii)

    • We can multiply the second equation by 2 and add equation (i) and (ii)
      o On multiplying equation (ii) by 2, we get 2a + 4b = 2m
      o On adding it with equation (i) which is a – b = n, we get:
         \(3a + 3b = (2m + n)\)
         \(a + b = \frac{(2m + n)}{3}\)
    • Now, we know don’t know if 2m + n is divisible by 3 or not. Hence, we cannot infer if a + b is an integer.
Hence, the correct answer is Option E.
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GMATBusters
If a > b, is (a + b) an integer?
Statement 1: (a - b) is an integer
Statement 2: a + 2b is an integer

GMATinsight's Solution


Question: Is (a + b) an integer?

Hint:

always question the information given in question e.g. in the given question the information about a and b to be integer is absent



Statement 1: (a - b) is an integer

case 1: a = 5 and b = 2, a+b = Integer
case 2: a = 5.2 and b = 2.2, a+b ≠ Integer

NOT SUFFICIENT

Statement 2: a + 2b is an integer

case 1: a = 5 and b = 2, a+b = Integer
case 2: a = 5.8 and b = 1.1, a+b ≠ Integer

NOT SUFFICIENT

Combining the two statements

a-b = Integer -------(1)and
a+2b = Integer-------(2)

Taking difference of the two equations i.e. (2) - (1)
3b = Integer

Taking Sum of the two equations i.e. (2) + 2*(1)
3a = Integer

i.e. 3a+3b = Integer
But a+b = Integer /3
Which may or may NOT be an integer hence

e.g.
case 1: a = 6 and b = 3, a-b and a+2b are both integer and a+b = Integer
case 2: a = 5/3 and b = 2/3, a-b and a+2b are both integer BUT a+b ≠ Integer


Answer: Option E
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