M2.2.6 For the adult
Estimate and check
Ravi worked out 487 + 312 and got 7 990. How can you tell he is wrong?
What this chapter teaches
An estimate is a careful guess: round each number, then work with the rounded ones. It lands near the right answer and shows a big mistake. To check a take away, add the answer back and look for the number you began with.
Three questions to ask
There is no one right wording. Listen for the idea.
- How could you tell quickly that 624 + 275 cannot be 1 899?
- How do you check a take away sum using adding?
- What would you round 487 and 312 to, and why?
After the chapter, the learner can
- Round numbers to estimate an answer before you work it out
- Check a taking away answer by adding back
What they copy into their notebook
Round, then work out a quick sum. 487 + 312 is about 500 + 300 = 800. To check a take away, add your answer back. You must get the first number.