Adding and Subtracting Within 100: practice solutions, Grade 1 – download the PDF

1 How many tens and ones? ★★★

a. There are 4 rods, so 4 tens. There are 6 dots, so 6 ones.

b. 4 tens and 6 ones is \(40 + 6 = 46\).

The number is 46.

2 Adding a few ones ★★★

The ones add up to less than 10, so the tens stay the same.

1. \(1 + 3 = 4\), so \(41 + 3 = 44\).
2. \(2 + 6 = 8\), so \(52 + 6 = 58\).
3. \(3 + 5 = 8\), so \(63 + 5 = 68\).
4. \(0 + 9 = 9\), so \(70 + 9 = 79\).

3 Adding tens ★★★

Add the number of tens.

1. 2 tens + 3 tens = 5 tens, so \(20 + 30 = 50\).
2. 4 tens + 4 tens = 8 tens, so \(40 + 40 = 80\).
3. 5 tens + 2 tens = 7 tens, so \(50 + 20 = 70\).
4. 1 ten + 6 tens = 7 tens, so \(10 + 60 = 70\).

4 Taking away tens ★★★

1. 9 tens − 3 tens = 6 tens, so \(90 - 30 = 60\).
2. 7 tens − 2 tens = 5 tens, so \(70 - 20 = 50\).
3. 6 tens − 2 tens = 4 tens, so \(60 - 20 = 40\).
4. 8 tens − 7 tens = 1 ten, so \(80 - 70 = 10\).

5 Sticker collection ★★★

We add: \(24 + 5\). The ones are \(4 + 5 = 9\), and the 2 tens stay.

\(24 + 5 = 29\)

Maya has 29 stickers now.

6 Hop on the number line ★★★

Count 4 ones from 35: 36, 37, 38, 39. You land on 39.

\(35 + 4 = 39\)

7 Ten more, ten less ★★★

10 more adds 1 to the tens digit. 10 less takes 1 away from it. The ones digit stays.

1. \(46 + 10 = 56\)
2. \(73 - 10 = 63\)
3. \(58 + 10 = 68\)
4. \(90 - 10 = 80\)

8 Make a ten ★★★

1. \(38 + 2 = 40\)
2. \(57 + 3 = 60\)
3. \(64 + 6 = 70\)
4. 5 is 2 and 3. First \(38 + 2 = 40\). Then \(40 + 3 = 43\). So \(38 + 5 = 43\).

9 Regroup the ones ★★★

1. \(9 + 4 = 13\) ones, so 1 new ten. \(2 + 1 = 3\) tens and 3 ones: \(29 + 4 = 33\).
2. \(7 + 6 = 13\) ones. \(4 + 1 = 5\) tens and 3 ones: \(47 + 6 = 53\).
3. \(8 + 7 = 15\) ones. \(5 + 1 = 6\) tens and 5 ones: \(58 + 7 = 65\).
4. \(6 + 9 = 15\) ones. \(3 + 1 = 4\) tens and 5 ones: \(36 + 9 = 45\).

10 Add a multiple of ten ★★★

Add the tens and keep the ones.

1. 3 tens + 2 tens = 5 tens, 4 ones: \(34 + 20 = 54\).
2. 6 tens + 3 tens = 9 tens, 1 one: \(61 + 30 = 91\).
3. 1 ten + 4 tens = 5 tens, 8 ones: \(18 + 40 = 58\).
4. 2 tens + 5 tens = 7 tens, 5 ones: \(25 + 50 = 75\).

11 Library cart ★★★

We add \(48 + 6\). The ones are \(8 + 6 = 14\), which is 1 ten and 4 ones. The tens are \(4 + 1 = 5\).

\(48 + 6 = 54\)

There are 54 books on the cart.

12 Find the mistake ★★★

Leo is not right. He added the ones \(6 + 7 = 13\) and then wrote 3 and 13 side by side. But 13 ones is 1 ten and 3 ones, so the ten must go with the other tens.

\(3 + 1 = 4\) tens and 3 ones, so \(36 + 7 = 43\).

13 True or false? ★★★

1. True: 4 tens + 3 tens = 7 tens.
2. False: \(5 + 4 = 9\), so \(25 + 4 = 29\).
3. True: 8 tens − 5 tens = 3 tens.
4. False: 6 tens − 2 tens = 4 tens, so \(60 - 20 = 40\).

14 Hop back ★★★

Hop back: \(90 \to 80 \to 70 \to 60\). Three hops of ten take away 30.

\(90 - 30 = 60\)

15 Two steps ★★★

Add the tens first because it is the easiest: \(28 + 30 = 58\). Then add the ones: \(58 + 4 = 62\).

\(28 + 30 + 4 = 62\)

16 Missing number ★★★

1. Count up from 34 to 39: 5. \(34 + 5 = 39\).
2. The tens go from 5 to 8, so 3 tens: 30. \(52 + 30 = 82\).
3. 47 needs 3 to reach 50. \(47 + 3 = 50\).
4. The tens go from 6 to 9, so 30. \(65 + 30 = 95\).

17 Mystery number ★★★

61 is 6 tens and 1 one. Adding 6 ones made a new ten, so the ones must have reached 11 before.

\(11 - 6 = 5\), so I started with 5 ones. My number is 55.

Check: \(55 + 6 = 61\).

18 Explain your strategy ★★★

\(46 + 8 = 54\).

Possible explanation: “I made a ten. 46 needs 4 more to reach 50. I split 8 into 4 and 4. \(46 + 4 = 50\), then \(50 + 4 = 54\).”

Another good explanation: “\(6 + 8 = 14\) ones, which is 1 ten and 4 ones. \(4 + 1 = 5\) tens, so 54.”

19 Farm stand ★★★

First add: \(40 + 30 = 70\) pieces of fruit. Then subtract the sold fruit: \(70 - 20 = 50\).

There are 50 pieces of fruit left.

20 Add 6 machine ★★★

\(24 + 6 = 30\). \(37 + 6 = 43\). \(58 + 6 = 64\). \(69 + 6 = 75\).

Each time the ones add up to 10 or more, we regroup and the tens digit goes up by 1.

21 Compare the sums ★★★

1. \(45 + 3 = 48\), and 48 < 50.
2. \(60 - 20 = 40\) and \(30 + 10 = 40\), so they are equal: =.
3. \(27 + 30 = 57\) and \(50 + 9 = 59\), and 57 < 59.
4. \(80 - 30 = 50\) and \(20 + 40 = 60\), and 50 < 60.