Curated OER
Add and Subtract within 20 - Independent Practice
Although this worksheet says it contains addition and subtraction within 20, it doesn't. Learners will have to add and subtract double-digit numbers with answers up to at least 30. However, it does provide good practice.
Khan Academy
Khan Academy: Subtract Within 20
Practice subtracting. All numbers in these problems are 20 or less. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a tutorial video, or receive hints.