Fluently add and subtract within 20 using mental strategies. See standard 1.OA.6 for a list of mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers.
Mentally add and subtract within 20. Use any strategy. Practice until all sums of one-digit numbers have been memorized. Next, give your students this challenge. Its only prereq is being able to mentally add single-digit numbers, with the sum being \(\le 24.\) After that, have students attempt this challenge. Next, lead this investigation to practice addition. Watch this to learn how. Finally, lead this investigation to practice subtraction. This video may help you.