Before you add multidigit numbers, you must memorize the results of
adding singledigit numbers, which are shown in the following table:
Addition Table

0 
1 
2 
3 
4 
5 
6 
7 
8 
9 
0 
0 
1 
2 
3 
4 
5 
6 
7 
8 
9 
1 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
2 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
3 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
4 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
5 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
6 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
7 
7 
8 
9 
10 
11 
12 
13 
14 
15 
16 
8 
8 
9 
10 
11 
12 
13 
14 
15 
16 
17 
9 
9 
10 
11 
12 
13 
14 
15 
16 
17 
18 
To add multidigit numbers, write the numbers down so that the
digits with the same place value are aligned in the same column.
Starting from the Ones place, add the digits in each column as you are
adding singledigit numbers. Work your way up to the highest place.
If the sum of the digits in one column is more than 10, then
carry over the face value in the Tens place of the sum to the next
higher column. Treat carryover as an addend for each column. Let us
illustrate how to add two multidigit numbers using an example.
Example:
what is the sum of 289 and 361?

First, we add the digits in the ones place : 9 + 1 = 10, so we need
to carry 1 over to the tens place and write down 0 in the ones place
of the sum. Then we add the digits in the tens place : 8 + 6 = 14 ,
do not forget the carry over : 14 + 1 = 15, so we need to carry 1
over to the hundreds place and write down 5 in the tens place of the
sum. Finally we add the digits in the hundreds place : 2 + 3 = 5, do
not forget the carry over : 5 + 1 = 6, so we just write down 6 in
the hundreds place of the sum. 
Adding three or more numbers can be done in the same way. You
could also add two numbers first and then add the sum to the third
number, and keep adding until all the numbers are added. An important
fact is that the order or grouping of numbers during addition does not
affect the result.
Subtraction is the opposite of addition. To subtract multidigit
numbers, write the minuend on top of the subtrahend and make sure that
the digits with the same place value are aligned in the same column.
Starting from the Ones place, subtract the digits in each column as
you are subtracting singledigit numbers. Work your way up to the
highest place.
If in some column, the minuend digit is less than the
subtrahend digit, then 1 needs to be borrowed from the next higher
column. Let us illustrate how to subtract two multidigit numbers
using an example.
Example:
what is the difference of 289 and 361?
Since 289 is less than
361, we are going to calculate 361  289 = ?

Firstly, we subtract the digits in the ones place. Since 1 is less
than 9, we need to borrow 1 from the tens place to make it 11. Now
11  9 = 2, just write down 2 in the ones place of the difference.
Then we subtract the digits in the tens place. Since 6 is less than
8, we need to borrow 1 from the hundreds place to make it 16. Now 16
 8 = 8, and we also have to subtract 1 from 8 because we borrowed 1
in the tens place before. Since 8 minus 1 is 7, just write down 7 in
the tens place of the difference. Finally we subtract the digits in
the hundreds place : 3  2 = 1, but we have to return 1 borrowed
before, so we got nothing left in the hundreds place of the
difference. 
Before you multiply multidigit numbers, you must memorize the results
of multiplying singledigit numbers, which are shown in the following
table:
Multiplication Table

0 
1 
2 
3 
4 
5 
6 
7 
8 
9 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
1 
0 
1 
2 
3 
4 
5 
6 
7 
8 
9 
2 
0 
2 
4 
6 
8 
10 
12 
14 
16 
18 
3 
0 
3 
6 
9 
12 
15 
18 
21 
24 
27 
4 
0 
4 
8 
12 
16 
20 
24 
28 
32 
36 
5 
0 
5 
10 
15 
20 
25 
30 
35 
40 
45 
6 
0 
6 
12 
18 
24 
30 
36 
42 
48 
54 
7 
0 
7 
14 
21 
28 
35 
42 
49 
56 
63 
8 
0 
8 
16 
24 
32 
40 
48 
56 
64 
72 
9 
0 
9 
18 
27 
36 
45 
54 
63 
72 
81 
Let us illustrate how to multiply multidigit numbers using an
example.
Example:
what is the product of 289 and 63?

Firstly, we multiply 289 by 3. Since 9 times 3 is 27, write down 7
in the ones place and carry 2 over to the tens place. Since 8 times
3 is 24, 24 plus 2 is 26, write down 6 in the tens place and carry 2
over to the hundreds place. Since 2 times 3 is 6, 6 plus 2 is 8,
write down 8 in the hundreds place. Then we multiply 289 by 60.
Since 9 times 6 is 54, write down 4 in the tens place and carry 5
over to the hundreds place. Since 8 times 6 is 48, 48 plus 5 is 53,
write down 3 in the hundreds place and carry 5 over to the thousands
place. Since 2 times 6 is 12, 12 plus 5 is 17, write down 7 in the
thousands place and carry 1 over to the tenthousands place. Since
there is no more digit to multiply, just write down 1 in the
tenthousands place. Finally, add the two rows, 867 and 17340 and we
get the answer : 18207 . 
Tricks and Tips 
When you multiply a whole number by
10, append one zero to that number. When you multiply a
whole number by 100, append two zeros to that number. When
you multiply a whole number by 1000, append three zeros to that
number.

Division is the opposite of multiplication. If your skills of
multiplication is rusty, you can not do division. There are two
methods for doing division: short division and long division. Long
division is more general. We will illustrate how to do long division
using an example.
Example:
how much is divided 3054 by 27 ?

Firstly, we find out that 3 is too small and we move the the next
digit 0. It is easy to see that 30 divided by 27 is 1 with remainder
3. Write down 1 in the hundreds place of the quotient and write down
the remainder 3. The next digit in the dividend is 5, so we are
going to divide 35 by 27, which should be 1 with remainder 8. Write
down 1 in the tens place of the quotient and write down the
remainder 8. The next digit in the dividend is 4, so we are going to
divide 84 by 27. Here we need to try out something. Since 2 times 27
is 54, which seems too smaller than 84, and 3 times 27 is 81, which
is smaller than 84 but very close to 84, we figure out that 84
divided by 27 is 3 with remainder 3. Write down 3 in the ones place
of the quotient and write down the remainder 3. That is it, 3054
divided by 27 gives a quotient 113 with a remainder 3. 
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