Cogito
Grade 2 Math · Chapter 4 · Lesson 7
Subtracting With a Broken Ten
Open a ten, then take away as usual.
10 problems · about 14 minutes · 2.NBT.B.5
What this lesson teaches
The child subtracts two-digit numbers that require unbundling one ten.
- Break a ten when there are not enough ones.
- Then take away as usual.
- Never swap the two digits round to make it easier.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Breaking a Ten Apart: 6 tens and 12 ones is which number?
Answer 72
Why 6 tens and 12 ones is 72.
Review — Subtracting Tens and Ones: 95 − 32 = ?
Answer 63
Why 95 − 32 = 63.
Warm Up
Straightforward practice. Get the method working first.
4 problems45 − 28 = ?
Answer 17
Why 45 − 28 = 17.
In 62 − 27, can you do 7 − 2 in the ones?
Answer No. You must take 7 from the 2.
Why Swapping them changes the question and gives the wrong answer.
52 − 38 = ?
Answer 14
Why 2 ones cannot lose 8. Break a ten.
84 − 29 = ?
Answer 55
Why 14 take 9.
Build It Up
The same ideas with more to keep track of.
2 problems76 − 43 = ?
Answer 33
Why No break needed here.
60 − 25 = ?
Answer 35
Why There are no ones at all. Break a ten.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Bake Sale: 73 cakes were baked and 48 were sold. How many are left?
Answer 25 cakes
Why 73 − 48 = 25.
Saving Up: Leo had 91 pence and spent 56. How much is left?
Answer 35 pence
Why 91 − 56 = 35.