I assume you have told her that to find the answer would be:
12/30 x 100
that is the simplest way I know. If she can lower the fraction it is often much easier, so it would become 2/5 which is obviously 40%.
Have taught DD a number of ways including the ways TIPSY suggests. I agree the best way, assuming the figures allow, is to simplify the fraction, not necessarily into it's lowest form, until the denominator is one that can easily be converted into percentages, e.g. 50,25,20,10,8,5,4,2.