Add and Subtract within 20 Mentally 2.OA.2 Add and Subtract within 20 Mentally Refer to standard 1.OA.6 Students are to use the Mental Math strategies listed in the first grade standard. 1.OA.6 Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8+6=8+2+4=10+4=14 ); decomposing a number leading to a ten (e.g., 13−4=13−3−1=10−1=9 ); using the relationship between addition and subtraction (e.g., knowing that 8+4=12 , one knows 12−8=4 ); and creating equivalent but easier or known sums (e.g., adding 6+7 by creating the known equivalent 6+6+1=12+1=13 ). Continue to build this fluency and honor the different ways students fluently solve and explain their thinking.

7 + 8 Add 1 I know that 7 + 7 = 14 and one more is 15 I know that 8 + 8 = 16 and one less is 16 I know that 8 + 2 = 10 so I will split 7 into 2 and 5. 10 + 5 = 15

8 – 3 Subtract I can count backwards 7,6,5 I know 8 – 4 = 4 so 8 – 3 = 5

Add 13 + 5 I will split 5 into 2 and 3. because 13 + 2 = 15 and 3 more is 18 I will count up 14,15,16,17,18

Subtract 13 – 5 I will split the 5 into 3 and 2. I will subtract 3 from 13 and get 10, then count back 2 more. so 13 – 5 = 8

Why did Kelly split up the numbers below? 12 + 7 10 2 Can you think of a way to solve this problem by splitting the 7?