“When you’re having trouble with math, what can you always count on for help? Your fingers!” For those elementary-age kids who can’t get enough of riddles, silly or otherwise, this series entry provides a deskful. While some are familiar chestnuts (“Why is 6 afraid of 7? Because seven ate nine!”), others are genuinely tricky and appropriate to the theme (“How many feet are in a yard? Twice as many as the number of people standing in it!”) and will provoke the occasional aha moment. Smith’s genially goofy illustrations accompany this equally amiable collection. (Early reader. 6-8)