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$f(x+y)=f(x)+f(y)$ for all positive integer values of $x$ and $y$. If $f(2)=4$, what is the value of $f(3)$?
General Hint: Enter the answer as an integer or a decimal in a single answer box. If two answer boxes are given, enter the answer as a fraction, with one box being the numerator and the other being the denominator. Read carefully whether the answer should rounded and make sure you round to the required degree of accuracy. Do not enter any symbols or characters in the box(es) provided.