Math Savant?

Abby has really been showing a special aptitude in math lately. It started this weekend, when she picked up a money workbook I'd found at the Dollar Store. She asked me to help her with it, and I explained that pennies were 1 cent and nickels were 5. With that information, she worked the first five pages in the book without my help. She could look at a picture of 3 nickels and 2 pennies and write 17, fairly quickly, without counting out loud or using her fingers. She just does it in her head. She did it again tonight. I bought some linking cubes to help Sarah count to 100 (Abby has been doing that for a long time, but Sarah still gets stuck). At the dinner table, I used 10 of them and asked a few math questions. I could put any number of them on the table, and then ask how many more I needed to make another number, and she'd get it immediately. Sarah didn't have a clue. I've always been good with numbers so I'm thrilled that Abby seems to enjoy math, and I hope with a little work we'll get Sarah on board soon!