If the numbers are increasing (getting bigger) you will either be adding or subtracting.
If the numbers are decreasing (getting smaller) you will either be subtracting or dividing.
Be sure to check the whole problem and not just look at the first few numbers to see what could be happening.
SHOW ALL OF YOUR WORK!
1, 3, 5, 15, 17
If you only look at the 1 and 3 you may assume that you adding 2. You could also be multiplying by 3! Check the whole pattern to find that the pattern is to multiply by 3 and then add 2.