a/b = c/d

where c is the answer blank. Of course if the formatting is as above then things are easy. Instead, I'd like to use \frac{ ans_rule(2) }{$d} for the right hand side.

I know I can use multianswer.pl but that seems to be overkill. Is there some quick way to embed a single answer blank into a tex formatted question using MathJax? I've tried tex_ans_rule and this gives

http://math.mc.edu/webwork2/PreAlgebra/C06-S02-Proportions/3/

Thanks,

JT