Most people would believe that the answer is 2 however answer is not 2. Because the equation is Y+1=3 with capital letter Y. And we have to find small letter y=?
We can find the Y as

Y + 1 = 3
Y = 3 - 1
Y = 2

Therefore Y = 2. We can find Y but can not find y.
Y and y are two different letters and there are two different variables. And we can find the Y by above solution (Y = 2) but we can not find y, so y is still unknown.

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