Decimal is the term most-often used to indicate that a number is not a whole number. In example 3.12, the decimal component is ".12." Keep in mind, however, that all numbers are decimals, but for convenience, the decimal is dropped if a number is a whole number. The number 3 is actually 3.0, and can also be written as 3.0, 3.00, 3.000000, etc. If you want to add or subtract decimals, line up the decimal points that you want to add or subtract and then simply perform the required operation. You may need to write in zeros in the places where such a 0 is implied and not really written. For example, 2.01+3.140=5.150 or 5.15.
To multiply decimals, multiply the two numbers as usual and then write the decimal point in the answer based on the total number of decimal places in the original decimal numbers. For example, if you want to multiply 2.3 and 4.2, first do the multiplication of 23 times 42, which gives you 966, and the total number of decimal places is two, one from 2.3 and one from 4.2, so add two decimal places to the answer to get 9.66.
To divide two decimals, set up a division problem as usual, count the number of decimal places in the divisor, and remove the decimal point in the divisor. Move the decimal point in the dividend to the right the number of decimal places that there were in the divisor. Perform the division as usual and give the answer the same number of decimal points as are in the dividend.