Virtual Manipulatives!

(from: https://www.commonsensemedia.org/app-reviews/virtual-manipulatives) VIRTUAL MANIPULATIVES is a solid math app with a focused purpose: to help learners visually understand the relationships among percentages, fractions, and decimals. Kids who use fraction bars/tiles at school will probably know what to do with the app and may find it to be a great homework helper. Otherwise, you'll need to explain the how and why for using this tool before kids will get much out of it. This is where the app falls short: It doesn't offer any guidance to parents on how to use the app with their kids.
But if you're able to meet that challenge, Virtual Manipulatives can make a tough concept a bit more easy to understand for kids with a variety of learning styles, with visual representations that kids can use their hands to manipulate. It's colorful, well designed, and free, making it a good addition to your math apps collection.