I very rarely promote books or products on Edgalaxy but I can personally and highly recommend this one by Kjartan Poskitt, after a parent in my grade gave me a copy to borrow.  As a result our school bought 10 to on sell in the office as it was just so practical.

Everyday Maths for Grown-ups is an excellent resource for teachers to give to parents to teach them how maths is taught in modern classrooms. And I recommend every school buying 10 or so copies to have in their office for parents to borrow or purchase.

Here is the speil from the publishers - but I can personallly say as a teacher it is a must have for confused parents.

Day-to-day life is full of scenarios where your skill with numbers is tested, whether it's dividing up your share of the restaurant bill, or working out whether you've been overcharged at the checkout. So many of us try to avoid these basic sums at all costs, waiting for someone else to step in with the answer; but "Everyday Maths for Grown-Ups" is the perfect solution! It includes chapters on: How to Check a Till Receipt Quickly; Long division; Multiplying and Dividing by 10, 100 or 1,000; Money and Percentages; Converting metres, Litres and Grams; and, Angles, Triangles and Trig. This book contains all the shortcuts, fun tricks and new approaches you need to tackle even complex calculations with confidence.

You can get it below via Amazon.  If you have any great recommendations we'd love to hear of them.