2.NBT.B.7
Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method. Understand that in 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.
Add and subtract within 1000, with and without regrouping. Practice exchanging 10 ones for 1 ten, 10 tens for 1 hundred when adding, and vice-versa when subtracting. Conclude by leading this investigation. Before doing so, you should watch this video too. It contains some solutions and some additional problems too.
Note: The Diffy Squares challenge is more appropriate for the lesson on adding and subtracting within 100. However, that topic has plenty of challenges already, so I have put Diffy Squares here, in the interest of sprinkling challenges throughout the curriculum instead of piling them all on one topic.