Answer
True Its not a well-defined function,
for example take $\frac{m}{n}$ = $\frac{1}{10}$, equviantally $\frac{m}{n}$ = $\frac{2}{20}$.
we get f($\frac{1}{10}$) = 1-10 = -9
while f($\frac{2}{20}$) = 2-20 = -18
hence its not a well-defined function since there is 2 outputs for the same input.
Work Step by Step
True Its not a well-defined function,
for example take $\frac{m}{n}$ = $\frac{1}{10}$, equviantally $\frac{m}{n}$ = $\frac{2}{20}$.
we get f($\frac{1}{10}$) = 1-10 = -9
while f($\frac{2}{20}$) = 2-20 = -18
hence its not a well-defined function since there is 2 outputs for the same input.