Addition & Subtraction Within 20 | No Prep 1st Grade Math Activities Make math practice FUN this St. Patrick’s Day with these engaging Crack the Code Math Worksheets! Students will use picture clues to solve addition, subtraction, and mixed problems within 20, then order numbers from greatest to least — building both computation and critical thinking skills at the same time. This NO PREP math packet is perfect for 1st Grade math centers, morning work, early finishers, homework, small groups