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The ideas in this decimals lesson are taken from Math Mammoth Decimals 1 book ($4.00 download). Only a few examples of each problem type are shown.

Adding decimal numbers - tenths
Free decimals lesson plan from HomeschoolMath.net

To learn how to add and subtract decimal numbers, try it yourself and compare to fraction addition!  Add the decimals.  Then write each decimal as a fraction, and add the two fractions.  Compare the answers.

Example problem types

1. Add mentally.

 
2.  Complete the following addition sentences.  Compare the top and bottom ones.

 
3.  Continue the patterns.

4.  Add and find what was added!

5.  Add/subtract in column form just like whole numbers, only line up all the decimal points and put a decimal point in the answer.

 
6.   Fill in the missing numbers.

7.  What is different between these two additions?

0.5 + 0.2 =

0.9 + 0.2 =

 
5.  Explain, using fractions, why the following addition is wrong:   0.7 + 0.8 = 0.15

8.  Word problems.

7. Two learning GAMES about adding/subtracting decimals with tenths.

You will need: 
- a big size number line from 0 to 5 with whole numbers marked on it, and tenths marked with small lines (tick marks).  The number line should be big enough that the play marks can be moved from one tenth to another.
- small play marks, one for each player
- cards with the numbers from the number line.  In other words, each card has one of the numbers 0, 0.1, 0.2, 0.3, ..., 4.7, 4.8, 4.9, and 5.0.

In the beginning each player places his/her play mark on the number line wherever desired but not on a whole number.
The winner is one who first visits each of the whole numbers 0, 1, 2, 3, 4, and 5.
At your turn, take a card from the stack.  You have to tell the difference between that number and where your play mark is currently located.  If you answer right, you can move your play mark the amount of the difference in either direction.  For example, let's say your play mark is at 3.4  and you draw a card with the number 2.8.  The difference is 0.6.  If you answer right, you can either move your play mark 0.6 up or down, and land either at 4.0 or at 2.8. If you answer wrong, the other players get a change - whoever answers the quickest.
If you cannot move the amount to your desired direction because the number line would end, then you simply move as far as you can go - either to 0 or 5.  For example, your play mark is at 4.7 and you draw a card with 3.1 on it.  The difference is 1.6.  In reality, moving up you would land at 6.3 which is outside the number line.  So you will move to 5 (or move down to 3.1) - which usually is a desired thing since you get to visit the whole number 5.  
Someone needs to keep track who has visited which whole numbers, or maybe players can collect something at each visit, like color-coded ribbons or stones etc. 

Variation: You will need the number line as above, play marks, and problem cards.  Making the problem cards can be a learning session, too.  Write decimal addition/subtraction problems where the answer is between 0.2 and 2.0, for example 2.0 + ____ = 2.3  or 0.5 + 0.7 = ____ or 1.3 - 0.5 = ____ etc.

In the beginning, each player puts his play mark at zero.  The winner is the one who reaches 5 first.
At your turn, take a problem card from the stack.  If you answer right, you can go forward as much as the answer was.  For example, the answer to the problem 2.0 + ____ = 2.3  is 0.3, so you can go forward three tenths on the number line.  In case of a wrong answer, whoever of the other players answers the quickest, can then go forward. (Variation: with wrong answer you go backwards 2 tenths.)  If at your turn, after answering right and forwarding your play mark, you land on a whole number, then an extra question is taken from the problem stack, and EVERYBODY gets a chance to answer that question, writing the answer on paper this time.

• Need more practice with decimal numbers?  Make free decimal number worksheets.

Next lesson: Multiply decimals with tenths

The ideas in this decimals lesson are taken from Math Mammoth Decimals 1 book ($4.00 download). Only a few examples of each problem type are shown.