Decimals Summary
A decimal number comes after the decimal point and tells you what fraction (small part) you have.
3.6 means 3 whole ones and 610.
3.06 means 3 whole ones and 6100.
3.006 means 3 whole ones and 61000 etc
Multiplying Decimal Numbers by 10, 100 & 1000.
· move the decimal point to the right.
· x10 move 1 place, x100 move 2 places, x1000 move 3 places etc
27.36 x 10 = 273.6
Dividing Decimal Numbers by 10, 100 & 1000.
· move the decimal point to the left.
· ÷10 move 1 place, ÷100 move 2 places, ÷1000 move 3 places etc
47.85 ÷ 1000 = 0.04785
Adding and Subtracting Decimals:
· Line up the decimal point
· Add or subtract as normal
Add 28.7 and 4.62
Make sure you line the decimal point up then just add as normal.
28.7
4.62
33.32
Multiplying Decimals:
· Ignore the decimal points and any zeros at the beginning or end of the numbers.
· Multiply the numbers as normal.
· Decide where to put the decimal.
Work out 2.542 x 0.02
Ignore the decimal points and do 2542 x 2 = 5084.
Now count how many numbers came after a decimal point in the question: 2.542 x 0.02 (there are 5 numbers after a decimal point: 5,4,2,0,2 (ticked) so you need to move the decimal point back 5 places so there are 5 numbers after the decimal point in your answer:
0.05084 (check your answer by counting how many numbers are after the point)
Dividing by a Decimal Number:
· Multiply the decimal by 10, 100 or 1000 to make it into a whole number.
· Do exactly the same to the number you are dividing into.
· Divide as normal.
Work out 5.16 ÷ 0.2 (we want to divide by 2, not 0.2 so x both bits by 10).
25.8
2)51.6
Work out 4.8 ÷ 0.03 (we want to divide by 3, not 0.03 so x both bits by 100).
160
3)480
Nb. You do not need to do anything to convert your answer – it is the final answer.
Rounding to a Certain Number of Decimal Places:
· Look at the next number after the one you want to stop at.
o 5 or above – round up
o Less than 5 – stays the same
Round 22.585 to 2 decimal places.
22.585 Answer is 22.59
Round 89.7714 to 3 decimal places.
89.7714 Answer is 89.771
Nb. Never move the decimal point. Never round down.
H Jackson 2010 / Academic Skills 1