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Adding decimal numbers - tenths
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3.5 + 1.5 =
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Remember that you are adding tenths, not just 'numbers' 8 and 4 (or whatever). If you have difficulty thinking that 0.8 + 0.4 would be 0.12, say the word "tenths" aloud with each addend. |
Example problem types
1. Add mentally.
| a. 0.6 + 0.5 =
b. 0.6 + 0.8 = c. 0.9 + 0.2 = d. 0.9 + 0.1 = |
m. 2.5 + 2.5 =
n. 1.2 + 1.2 = o. 3.5 + 2.5 = p. 10.9 + 1.1 = |
2. Complete the following addition sentences. Compare the top and bottom ones.
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3. Continue the patterns.
| a.
0.6 + ___ = 1.0 0.6 + ___ = 1.1 0.6 + ___ = 1.2 0.6 + ___ = ___ 0.6 + ___ = ___ 0.6 + ___ = ___ 0.6 + ___ = ___ |
d.
3.5 + ___ = 3.0 3.5 + ___ = 2.9 3.5 + ___ = 2.8 3.5 + ___ = ___ 3.5 + ___ = ___ 3.5 + ___ = ___ 3.5 + ___ = ___ |
4. Add and find what was added!
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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.
| a. | b. | f. |
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17.8
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50.2
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| g. | h. | l. |
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50.2 |
157.7 |
60.3 |
6. Fill in the missing numbers.
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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.
a. Sandy weighs herself every month during her pregnancy.
How much did she gain each month?
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| b. Normal body temperature is considered to be 98.6 degrees
Fahrenheit. How many degrees above that is your temperature, if it
is 99.9F? 100.4 F? 102.2F?
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| f. In the school's swimming competition, Marsha gets the time 54.4
seconds. The winner is 0.6 seconds quicker. What was the
winning time?
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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.
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How many different solutions can you find for this puzzle?
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Next lesson: Multiply decimals with tenths
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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. |
The fast, FUN, and easy way to learn multiplication. Learn the upper times tales in two sittings using mnemonic stories.
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