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Level 6: AddDecimal

8/13/2015

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Adding decimals is almost as easy as adding whole numbers.  We need to pay extra attention to place values, and we must never write extra zeros, which is a waste of time.

Let us try 6.7 + 5.3.  Regrouping, we have (6 + 5) ones plus (7 + 3) tenths.  Simplifying, we have 11 ones + 10 tenths, or 12 ones.  The answer is 12.

Now let us try 1.6 + 2.1 + 12.3.  Regrouping, we have 1 ten, plus (1 + 2 + 2) ones, plus (6 + 1 + 3) tenths.  Simplifying, we have 1 ten + 5 ones + 10 tenths, or 10 + 5 + 1.  The answer is 16.

Let us try 17.23 + 1.18 + 10.39.  Working from right to left, we have (3 + 8 + 9) = 20 hundredths.  We do not write the zero in the hundredths place.  We regroup the 2 with the tenths.

So (2 + 1 + 3) plus the regrouped 2 tenths = 8 tenths.  We write an 8 with a decimal to its left.

Then we have (7 + 1 + 0) ones, or 8 ones.  We write the 8 to the left of the decimal.

Then we have (1 + 1) tens, or 2 tens.  We write the 2 at the left.  The answer is 28.8.

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    I competed in Number sense in the early 1990s.  Now, I'm coaching the next generation of superstars.  Hopefully, this site will be helpful to you.

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