I'm trying to write some simple calculated MC questions, but can't get the syntax for the answers down. Let's say my question looks like:

What is $\frac{a}{b} + \frac{x}{y}$ ?

One of my answer choices might be:

{=$\frac{{{a}+{x}}}{{{b}+{y}}}$}

Or one of the hundred subtle varients I've seemingly tried. My typical kick-back from the quiz engine is:

Illegal formula syntax starting with '$\frac{{':$\frac{{1+1

And then I'm just lost. I've been through the math and TeX documentation and I guess since none of the examples I found used any TeX I'm a little lost. Any help would be greatly appreciated! Thanks

Btw, I am writing double $ in all of my TeX, just not here to keep the TeX filter from changing it.