I was debating with a maths teacher about how to simplify algebraic fractions, and what their simplest form is. From the starting point of $\frac{100x+t(300-x)}{300}=75$, you can easily simplify down to $x=\frac{22500-300t}{100-t}$. This is where they advocated leaving it, as it's supposedly in simplest form. I think, however, that even though it can't be truly simplified, turning it into something such as $x=300-\frac{7500}{100-t}$ is much better, even if it does introduce an extra term.
It's important to note that this has nothing to do with marking. I learnt algebraic fractions five or six years ago. This is just a fun little debate about which form should be considered simplified.