The next number should be 4. If you look at every other number, even and odd alternate. 2 is even, 7 is odd, etc. Since 9 is odd, the next number should be even. To figure out what number specifically, you will want to break up the numbers into three columns. 2 3 __ 7 12 17 8 9 10 From here, you can see that the pattern will be adding 1 for the top row, 5 for the middle row, and 1 for the bottom row. Since the unknown number is on the top right of the first row, you will add 1 to 3, and the answer is 4.