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kkannan2
Hi CrackverbalGMAT - not understanding your explanation here. How do you move from M=9n to substituting that in equation (3), which is 2I=M-n, and re-writing that as 2I=(7*9n)-n? Where does the 7 come from? Wouldn't it now be 2I=9n-n=8n, so I=4n, in which case the answer would be 12, as that's the only multiple of 4 in the answer choices? I'm confused how you made this leap to multiply 9n by 7 when plugging into equation (3). Please clarify.­
There is a typo in that solution. Here is how to do it.

Adding ­I + n = 4 * (M - n) and I - n = 3 * (M + n) gives 2I = 7M - n

Subtracting I - n = 3 * (M + n) from I + n = 4 * (M - n) gives 2n = M - 7n, which leads to 9n = M

Put 9n = M into 2I = 7M - n to get 2I = 7(9n) - n = 62n, which gives I = 31n.
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