15, it is the only odd number that is counted by 5, less than 20 and is odd. You can explain it two your daughter like this (not sure whether you meant 2nd grade or 2 years old):
You start on 5, then you count up by 5 once. You get 10. 10 is an even number so you count up by 5 again. You get 15. Before deciding 15 is your answer you count up by 5 once more,. You get 20. 20 is not less than 20, so you go back to 15. 15 is your answer.
