Decimals are a method of writing fractional numbers without writing a fraction having a numerator and denominator.
The fraction 3/10 is written as the decimal 0.3.

Decimals are fractional numbers. The decimal 0.3 is the same  and equal to fraction 3/10,the number 0.23 is a decimal that represents 23/100.
Adding Decimals is just like adding other numbers.in simple adding operation
Always line up the decimal points when adding decimals. means point of decimal are lie in same place like

                    22.33 add 2.3 then written as
                         22.33        +
                           2.3
                     .....................
                       24.63
                  ........................

Remember to put the decimal point in the proper place in your answer.

Decimal numbers, such as O.8766, have fiver digits after the decimal point.  Each digit is a different place value.  as 8 7 6 6
The first digit after the decimal point is called the tenths place value.

The second digit tells you how many hundredths there are in the number. 

          The third digit is the thousandths place. 

The fourth digit is the ten-thousandths place . 

Rounding decimals is very similar to rounding other numbers. If the thousandths place of a decimal is four or less, it is dropped and the hundredths place does not change. For example, rounding 0.843 to the nearest hundredth would give 0.84.
If the thousandths place is five through nine, the hundredths place is increased by one. The decimal 0.846 rounded to the nearest hundredth is 0.85.