2.NOS.C.9: Add and subtract within 1000, recognizing that when adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds. a. Use concrete models or drawings to add and subtract. b. Use strategies based on place value (counting on, partial sums, making tens, etc.) to add and subtract. c. Use properties of operations, and/or the relationship between addition and subtraction to add and subtract. d. Represent and explain the calculation by connecting the strategy used to the meaning of addition and subtraction.