"A Number String is a set of related math problems designed to teach strategies based on number relationships. It is a 10–15 minute routine that can be used during math instruction." (From: https://www.hand2mind.com) See Hand to Mind website for more information. These are meant to do synchronously, with students sharing their thinking aloud to build number sense by finding those relationships between familiar facts to those that are unknown. Below are some different examples of what you may use with 3rd graders.