Addition Worksheets for 1st Graders Online - SplashLearn Mastery in addition facts is now super easy! 1st graders can thoroughly practice in the fluently add within 10 with this worksheet. K 1. Fluently Add within 20. Add and Subtract 1-Digit Numbers within 20: Horizontal Addition and Subtraction. Nurture your 1st graders' addition facts skills with this fun worksheet.