You are turning the wrong fraction upside down :)
3/5n - 4/5 = 1/5n
I'll show you very formally.
The equation must always balance, like a tetter totter (I think that is what Americans call them)
Whatever you do to one side you must do to the other side as well!
\(\frac{3}{5}n-\frac{4}{5}=\frac{1}{5}n\\~\\ \frac{3}{5}n-\frac{4}{5}-\frac{1}{5}n=\frac{1}{5}n-\frac{1}{5}n\\~\\ \frac{2}{5}n-\frac{4}{5}+\frac{4}{5}=0+\frac{4}{5}\\~\\ \frac{2}{5}n=\frac{4}{5}\\~\\ \frac{2}{5}n\div \frac{2}{5} =\frac{4}{5}\div \frac{2}{5}\\~\\ n=\frac{4}{5}\div \frac{2}{5}\\~\\ n=\frac{4}{5}\times \color{blue}{\frac{5}{2}}\\~\\ n=\frac{2}{1}\\~\\ n=2\)
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