Dear Wendy,
Thanks for sharing your question. You as her Mom can best judge if she is really getting the concept of not. The idea of mental math is to do it mentally, but it sounds like her hesitation might mean she is still having trouble visualizing the abacus or perhaps she doesn't have strategies yet for solving mental problems. I would recommend that you let her write it down if she needs to, but I would rather she actually enter the numbers on the second side of the abacus and practice with that so she is visualizing what she is doing. In the problem you mentioned she should have the strategy of competing the ten. for instance, add one to the 39 to make it a 40 by taking a one from the 54 making it 53. Perhaps you should share your strategy with her. If you are not clear on the strategies yourself they are listed in the back of the Math Card Games Book. It also helps to play more math games so she becomes more automatic in her mental computation. The Corners game helps mental adding. How often do you play the math games?
Nancy