Decimals
Rules for Adding,
Subtracting, Multiplying,
Dividing, and Rounding
Adding Decimal Numbers
• Adding decimals is very much like adding whole
numbers.
• The most important thing to remember when
adding decimals is to line up all the decimal
points in a column!
• You can put in 0’s as place holders if you would
like to help keep the columns straight, but it’s not
necessary.
• Place the decimal point in the answer directly
below the decimal points in the terms.
Adding Decimal Numbers
Subtracting Decimal
Numbers
• Put the numbers in a vertical column lining up
the decimal points.
• You must put in 0’s as place holders to help keep
the columns straight, but also so you’ll know if
you need to borrow.
• Subtract each column, starting on the right and
working left. If the top number is smaller than the
bottom number, borrow from the top number at
the left.
• Place the decimal point in the answer directly
below the decimal points in the terms.
Subtracting Decimal
Numbers
Multiplying Decimal
Numbers
• Put the numbers so that the last digits line up,
without trying to line up the decimal points.
• Do NOT put in 0’s as place holders.
• Multiply as if the numbers were whole numbers.
• Determine the TOTAL number of digits to the
right of the decimal points in the numbers.
• Place the decimal point in the answer so the
number of digits to the right of the decimal point
is the same as the TOTAL number of digits
above.
Multiplying Decimal
Numbers
Dividing Decimal Numbers
• Multiply both the dividend and divisor by a
power of 10 that will make the divisor (number
you are dividing by) a whole number (or just
move the required number of decimal places,
filling in 0’s where needed).
• Divide as if working with whole numbers.
• Place the decimal point in the answer directly
above the decimal point in the dividend.
Dividing Decimal Numbers
Rounding Decimal
Numbers
• Identify the number in the problem to the right of the
positional value to which the number is to be rounded
– For example, if you wish to round to tenths, you identify the
number in the hundredths position.
• If the number you identified above is greater than or
equal to 5, the number in the preceding position is
increased by 1 unit and all numbers to the right are
eliminated.
• If the number you identified above is less than 5, the
number in the preceding position stays as it is and all
numbers to the right are eliminated
Rounding Decimal
Numbers