This Number Talk is designed to develop fluency with addition and subtraction of multi-digit numbers. This Warm-up also gives students a chance to reason about numbers beyond 1,000. The understanding elicited here will be helpful later in the unit and throughout IM Grade 4 when students add and subtract fluently using the standard algorithm.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Teacher Instructions
Record answers and strategy.
Keep expressions and work displayed.
Repeat with each expression.
Student Task
Find the value of each expression mentally.
650+75
5,650+75
50,650+75
500,650+75
Sample Response
725: 75 is also 50+25 and 650+50=700 and 700+25=725
5,725: Five thousands is the only new part of this number and 75 is not going to change the thousands. So, the answer is 5,725.
50,725: Fifty thousand is the only new part of this number and 75 is not going to change the ten-thousands. So, the answer is 50,725.
500,725: Five-hundred thousand is the only new part of this number and 75 is not going to change the hundred-thousands. So, the answer is 500,725.
Synthesis
“Which parts of the number change when we add 75?” (Just the hundreds, tens and ones.)
“How do we know without adding if digits in a number are going to change?” (Because we are not adding more than 10 tens to any of the numbers and there's only 6 hundreds. We know the thousands are not going to change.)
Standards
Addressing
4.NBT.4·Fluently add and subtract multi-digit whole numbers using the standard algorithm.
4.NBT.B.4·Fluently add and subtract multi-digit whole numbers using the standard algorithm.