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...
I feed my hermit crab whenever her shell is empty, but shje never eats more then 1 pellet every 3 days. Normally she only eats one a week. Each pellet is about the size of a fine sharpie marker. Is this normal for her to eat so little?