Numeracy Ninjas is a free UK Math resource for their "Key Stage 3 numeracy intervention designed to fill gaps in students’ basic mental calculation strategies and also to empower them with the numeracy skills and fluency required to fully access GCSE Maths concepts when they move to Key Stage 4 study." This resource can easily be adapted for use in the US for intervention needs here.
The resources listed below are just some of what is available on the Ninjas Website.
Ninja Skill Book for 3-Day Lessons 1-10
Ninjas Skill Book for 3-Day Lessons 11-20
Ninja Skill Book for 3-Day Lessons 21-30
Ninja Skill Book for 3-Day Lessons 31-40
The Answer Key for class use
Where to keep track of student progress