Decimal Multiplication & Division Mathematics Decimal Multiplication & Division

The aim of this powerpoint is to teach you how to multiply and divide decimal numbers using skills you have learnt in previous powerpoints. EITHER Take notes as you go along, include some examples and write down any questions and your answers (which you can mark as you go along) OR At the end of the powerpoint, printout the notes called Calc7d

Multiplying Decimals Imagine the values without decimal points Multiply them together Count the number of digits that came after the points in BOTH of the original numbers in the calculation From the right to left, count that many digits into your answer and write in your decimal point Check your answer makes sense using rounded values for the original values in your head!

Example 1 What is 3.8 × 0.4 ? Calculate: 38 × 4 (using any short multiplication method) Answer: 38 × 4 = 152 In the original calculation 3.8 × 0.4, 2 digits came after the decimal points Count these back into our answer and then write the decimal point 152  1.52 Mental check: 4 × 0.4 = 1.6 [or 4 lots of ½ = 2]

Example 2 What is 2.38 × 1.2 ? Calculate: 238 × 12 (using any long multiplication method) Answer: 238 × 12 = 2856 In the original calculation 2.38 × 0.4, 3 digits came after the decimal points Count these back into our answer and then write the decimal point 2856  2.856 Mental check: 2 × 1 = 2

Practice Work through the MyMaths lesson (and then its online homework) called Multiply Decimals by Whole Numbers found at: http://www.mymaths.co.uk/tasks/library/loadLesson.asp?title=decimals/decimalMultiplying1&taskID=1010 http://www.mymaths.co.uk/tasks/library/loadTask.asp?title=decimals/decimalMultiplying1OH&taskID=1010 Work through the MyMaths lesson (and then its online homework) called Multiply Two Decimals found at: http://www.mymaths.co.uk/tasks/library/loadLesson.asp?title=decimals/decimalMultiplying2&taskID=1011 http://www.mymaths.co.uk/tasks/library/loadTask.asp?title=decimals/decimalMultiplying2OH&taskID=1011 Save and complete the worksheet called: MultDec-S1.xlsx

Dividing Decimals Find an equivalent division where the divisor (the second number) is a whole number. It does NOT matter if the first number is still a decimal! Sometimes you can double both numbers to find an equivalent calculation but more often than not you will need to multiply them both by 10, 10, 1000 etc. (to remove the decimal point!) Carry out your preferred method of long division on the equivalent calculation. The answer to this is the SAME as the answer to the original calculation (as both calculations are equivalent – i.e. worth the same!)

Example 1 What is 52 ÷ 0.8? Multiply BOTH values by 10 to get: 520 ÷ 8 Use your chosen method of long division to find the answer to this… 6 5 5 4 8 5 2 0 The answer to our original calculation is also 65 Check using inverse op: 65 x 0.8 = 52 OR using estimation: 52 ÷ 1 = 52 (we were dividing by a smaller amount so our answer should be bigger than this)

Example 2 What is 2.5372 ÷ 0.04? Multiply BOTH values by 100 to get: 253.72 ÷ 4 Use your chosen method of long division to find the answer to this… 6 3 . 4 3 2 1 1 1 4 2 5 3 . 7 2 The answer to our original calculation is also 63.43 Check using inverse op: 63.43 x 0.04 = 2.5372 OR using estimation: 2 ÷ 0.04 = 50 (we were dividing a larger amount than 2 to start with so our answer should be bigger than this)

What next? If you haven’t made any notes or copied any examples, questions and answers out during this presentation, print out the notes called Calc7d. Read through them and make sure you answer any questions. Work through the MyMaths lesson (and then the online homework) called Divide Decimals by Whole Numbers found at: http://www.mymaths.co.uk/tasks/library/loadLesson.asp?title=decimals/decimalDividing&taskID=1008 http://www.mymaths.co.uk/tasks/library/loadTask.asp?title=decimals/decimalDividingOH&taskID=1008 Save and complete the worksheet called DivDec-S1.xlsx Now move on to the Calc8-Negatives powerpoint