I am attempting to generate formula questions which will generate random fractions. I would prefer the fraction look a bit " cleaner" so I am using the fraction tool in the equation editor. The problem is I cannot get it to show the randomly generated numbers

\frac{[a] }{[b] }x+[c]=[d]

c and d show up fine.... a and b do not show up. This is the advanced fraction command in the equation editor. Anyone have any ideas?