This will be problems such as 40 + 3, 60 + 20, 42 + 7, 34 + 20.
e. g. 40 + 3
To solve a problem like 40 + 3, use the system of numeration.
40 = 4 tens
3 = 3 ones
4 tens + 3 ones = 43 ones
This is called the system of numeration [addition and place value]
So, 40 + 3 = 43

Solve these problems using the above method
Practice 1:
20 + 8
20 = 2 tens
8 = (  ) ones

Correct
Wrong
 2 tens + (  7 8 9 ) ones = (  27 28 29 ) ones Correct Wrong Correct Wrong

Practice 2:
60 + 7

 (  5 6 9 ) tens + (  9 7 8 ) ones = (  69 67 68 ) ones Correct Wrong Correct Wrong Correct Wrong

(67) ones = (67)

Solve Mentally:
1. 30 + 3
2. 20 + 6
3. 90 + 1
4. 40 + 8

e. g. 60 + 20
To solve a problem like 60 + 20, take the tens digits as ones and add them.
60 ones = 6 tens
20 ones = 2 tens
6 tens + 2 tens = 8 tens
8 tens = 80
So, 60 + 20 = 80

Solve these problems using the above method
Practice 1:
40 + 30
40 ones = 4 tens
30 ones = (  ) tens

Correct
Wrong
 4 tens + (  1 3 2 ) tens = (  6 7 8 ) tens Correct Wrong Correct Wrong
 (  5 7 8 ) tens = (  77 70 72 ) ones Correct Wrong Correct Wrong

Practice 2:
50 + 40

 (  6 5 9 ) tens + (  3 4 5 ) ones = (  7 9 10 ) tens Correct Wrong Correct Wrong Correct Wrong

 (  10 9 8 ) tens = (  87 90 89 ) ones Correct Wrong Correct Wrong

Solve mentally:
1. 30 + 40
2. 60 + 20
3. 10 + 80
4. 40 + 50

e. g. 42 + 7
To solve a problem like 42 + 7, split the 42 into ones and tens and add the ones together. Then use system of numeration to add tens and ones.
42 = 40 + 2
2 + 7 = 9
40 + 9 = 49
So, 42 + 7 = 49

Solve these problems using the above method
Practice 1:
42 + 6

 42 = (  41 40 42 ) + (  1 2 4 ) Correct Wrong Correct Wrong

 (  4 2 3 ) + 6 = (  7 8 9 ) Correct Wrong Correct Wrong

 40 + (  9 8 7 ) = (  47 48 49 ) Correct Wrong Correct Wrong

Practice 2:
73 + 5

 73 = (  71 70 69 ) + (  3 2 4 ) Correct Wrong Correct Wrong

 (  4 5 3 ) + (  7 5 6 ) = (  8 7 9 ) Correct Wrong Correct Wrong Correct Wrong

 (  71 70 69 ) + (  7 9 8 ) = (  77 78 79 ) Correct Wrong Correct Wrong Correct Wrong

Solve Mentally:
1. 33 + 3
2. 67 + 2
3. 71 + 8
4. 91 + 7
5. 57 + 2
6. 83 + 6

e. g. 34 + 20
To solve a problem like 34 + 20, split the 34 into ones and tens and add the tens. Then use system of numeration to add tens and ones.
34 = 30 + 4
30 + 20 = 50
50 + 4 = 54
So, 34 + 20 = 54

Solve these problems using the above method.
Practice 1:
42 + 30

 42 = (  40 41 39 ) + 2 = (  42 40 39 ) + 30 = (  72 70 69 ) Correct Wrong Correct Wrong Correct Wrong

 (  72 68 70 ) + 2 = (  71 72 73 ) Correct Wrong Correct Wrong

Practice 2:
43 + 50

 43 = (  40 38 43 ) + (  4 3 2 ) Correct Wrong Correct Wrong

 (  39 38 40 ) + (  40 50 60 ) = (  70 90 80 ) Correct Wrong Correct Wrong Correct Wrong

 (  90 93 91 ) + (  5 3 2 ) = (  97 92 93 ) Correct Wrong Correct Wrong Correct Wrong

Solve Mentally:
1. 34 + 30
2. 57 + 20
3. 12 + 80
4. 21 + 70
5. 43 + 20
6. 53 + 30