# ADDITION AND SUBTRACTION OF DECIMALS

LESSON PLANNING OF ADDITION AND SUBTRACTION OF DECIMALS

Subject Mathematics

Students` Learning Outcomes

Information for Teachers

• If you know how to add and subtract whole numbers, then you can add and subtract decimals. Just be sure to line up the terms so that all the decimals points are in a vertical line, on top of each other.  For example
• If addenda have different number of decimal places, we equate their decimal places by writing zeros (0) at end where required. For example; 31.80 + 00.45 will be written in the following way as;
• Put the numbers in a vertical column, aligning the decimal points/
• Add each column of digits, starting on the right and working left. If the sum of a column is more than ten, “carry” digits to the next column on the left.
• Place the decimal point in the answer directly below the decimal points in the terms.
• To subtract decimal numbers:
• Put the numbers in a vertical column, aligning the decimal points, as you did for addition.
• Subtract each column, starting on the right and working left, if the digit being subtracted in a column is larger than the digit above it; “borrow” a digit from the next column to the left.
• Place the decimal point in the answer directly below the decimal points in the terms.
• Check your answer by adding the result to the number subtracted. The sum should be equal to the first number.
• Consult the textbook when and where required.

Material / Resources

Writing board, chalk / marker, duster, one loose sheet per child, textbook, mixed ability group-wise class list

Introduction

• Tell students to clear their desks and ask what we have in front of us. [Nothing or zero]
• Draw on the board a number line and write 0 on one side.
• Ask to take out one loose sheet and place it on the desk.
• Ask ‘how many sheets do we have in front of each of us? [One]
• Write 1 on the other side on the number line.
• Ask, what do we have between 0 and 1? [They might reply that few parts or fractions or decimals or nothing]
• Introduce that today we will explore about numbers between two whole numbers on a number line.

Development

Activity 1

• Cut out a paper sheet into 10 equal strips.[as few strips are shown in the figure].
• Ask how many strips do we have? [10]
• Ask what do you call each one of it? Is it zero or is it as big as to be called one whole sheet? [students would agree to say no, it`s not the whole sheet]
• Ask: so what do we call each of the strips? [Let them think] then repeat the concept of fraction that “one strip out of 10” means [1/10]
• [1/10] is also called 0.1
• Make a number line from 0 to 10.
• Draw 10 lines on the number line and say, if one strip is not the whole number 1 then can we call it 0.1? [Some would say yes or some would still be confused]
• Let`s have a look at that…., if one strip is 0.1 then put all the strips alongside and see if it make one whole sheet. [Yes]
• Write vertically on board and add with the whole class.
• Hence conclude that 10 strips of 0.1 when kept alongside make one whole sheet.
• Fill the number line on the board like this. Show with arrow the plus sign.
• One strip = 0.1
• Now cut each 0.1 strip into 10 equal small squares.
• Now what do we call each small square, is it 0.1? [No, as 0.1 is one strip which is bigger than this small square]
• Let`s check it out, if we call each of the square as 0.01, does ten of them makes 0.1 or one whole strip or not?
• Add vertically on board 0.01 [ten times]
• Do it on the board and show that ten of 0.01 when added makes 0.1. [this will make them learn addition of decimals along with the revision of decimal concept]
• Erase the board, let students have those strips in their hands and ask following review questions: if 0.1 = one strip, then

o   0.1 + 0.1 =? [two strips, 0.2]

o   0.2 + 0.1 =? [three strips, 0.3]

o   0.2 – 0.1 =? [two strips minus one, 0.1]

o   0.5 + 0.1 =? [six strips, 0.6]

o   0.5 – 0.1? [four strips, 0.4]

o   0.6 + 0.4 =? [six strips + four strips is equal to , 1.0, one whole sheet]

o   0.6 – 0.4 =? [six strips minus four strips, 0.2, two strips]

• They will learn addition and subtraction of decimals.
• Write the following on the board:
• Explain the placement of decimal under decimal as explain in the information for teachers. For example;

Activity 2

• Split the class into groups of mixed ability. [it is good if the groups are decided in the beginning of the unit and hence the work together for all the lessons of the unit. The members of mixed ability should be grouped by the teacher]
• Give questions from the textbook and allocate specific time limit.
• Give the time to the students to do extensive practice of the addition and subtraction of decimals and discuss them in pairs.
• Discuss few questions as a whole class; invite students in pairs to explain the method of the addition and subtraction of decimals on the board.
• Involve the passive students, as well.
• Group work followed by class discussion ensures the understanding and application of skills. Now is the time for individual assessment.

Activity 3

• Individual work:
• Write few questions of the addition and subtraction of decimals on the board.
• Allocate specific time to the students to complete their work individually.
• Collect notebooks to assess later.

Sum up / Conclusion

• The word “Decimal” really means “based on 10” (from Latin decimal: a tenth part).
• We sometimes say “decimal” when we mean anything to do with our numbering system, but a “decimal number” usually means there is a decimal point.
• Explain with different examples, such as;

o   If the petrol price was Rs, 125.55 and then there was an increase of Rs.3.5. what`s the new price?

o   A baby`s weight was 5.5 pound which then increased to 8.5 pounds. How much was it increased?

• Tell them that similarly the decimals are used everywhere, calculating distance from moon or stars to buying grocery from the store.
• Conclude the discussion on that 0.1 is also 1/10, 0.5 is also 5/10 etc. [may relate with the paper cutting activity done in the beginning]

Assessment

• Write following questions on the board and ask them to solve on their copies; e.g. (4.50 + 0.8), (2.087 + 4.92) and (4.153 – 2.374)
• Line up the decimal points and fill in zeros (0) to give the same number of decimal places before adding or subtracting. Now solve them:

• Questions from the textbook;
• Ask students to look at a ruler and see how a cm is divided into smaller parts too, just like the numbers on a number line.
• Solve the following word problems: