See the yellow 5? Swipe that up or to the right, and it will divide into 3 & 2. Other rules: you can't add above 9. A single 1 cannot move to a vacant circle. See the 4 & 2 in the first image? If you swipe from the 4 to the right, you'll get 6 on the yellow circle.
There are 48 free levels. I've solved them all - pretty quickly. However, I have not solved them all using the least number of moves. You get a star if you solve a level with the least # of moves. The leaderboard will only record your score based on your stars. At the moment, I have 36 stars.
There are another 72 levels and they cost $.99. Noda has a serious number of players, based on the leaderboard, so it's quite popular. Because it's fun, easy to learn with good graphics.
Update 12/15: I have now solved all the free levels & have 48 stars.