Cogito
Grade 2 Math · Chapter 4 · Lesson 8
Checking by Adding
Add your answer back and see if you return.
10 problems · about 12 minutes · 2.NBT.B.9, 2.OA.A.1
What this lesson teaches
The child checks a subtraction by adding the answer to the number taken away.
- Adding undoes subtracting.
- Add your answer to what was taken away.
- If you get back to the start, you were right.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Subtracting With a Broken Ten: 45 − 28 = ?
Answer 17
Why 45 − 28 = 17.
Review — Breaking a Ten Apart: 6 tens and 12 ones is which number?
Answer 72
Why 6 tens and 12 ones is 72.
Warm Up
Straightforward practice. Get the method working first.
4 problems75 − 38 = ?
Answer 37
Why 75 − 38 = 37, and 37 + 38 = 75.
How do you check a subtraction?
Answer Add the answer to what was taken away.
Why If it returns the starting number, the answer was right.
61 − 34 = ?
Answer 27
Why Break a ten.
Check it: what is 27 + 34?
Answer 61
Why It should come back to 61.
Build It Up
The same ideas with more to keep track of.
2 problems90 − 47 = ?
Answer 43
Why No ones at all. Break a ten.
Check it: what is 43 + 47?
Answer 90
Why It should come back to 90.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsChecking Up: Ana says 84 − 29 = 55. Check it: what is 55 + 29?
Answer 84
Why It returns 84, so her answer was right.
A Wrong Answer: Leo says 52 − 38 = 24. What is 24 + 38?
Answer 62
Why It gives 62, not 52, so his answer was wrong.