Handling the Decimal Point

Do you know of a way we can explain the placement of the decimal point in multiplication to our daughter so it is more than a mechanical operation? -- Concept Lacking
Answer: Find a set of multiplication problems involving decimals like 34.7 x 12.38 in your child's math textbook. Using a calculator, have her find the product ignoring the decimal points. In this problem, she would get 429586.
Ask your child to locate the correct position of the decimal point in the answer. To do this, have her estimate the answer by rounding each factor to the nearest one and multiplying the new factors. For our example: 35 x 12 = 420. Suggest that the decimal be placed in 429586 to get as close as possible to 420 (the product of 35 x 12). The decimal point is at 429.586.
After your child has worked through a number of similar problems, ask her to make up a rule for locating the decimal point in multiplication. It is placed so that the number of digits to the right of the point equals the number of places in the factors. For example, there are three decimal places in the problem 34.7 x 12.38, so she should place the decimal three places from the right of the product: 429.586. If your daughter still has difficulty understanding the rule for decimals in multiplication, have her work with smaller numbers.
This explanation is from our book "Helping Children With Mathematics." The book has activities to help children in grades 3-5 learn basic math concepts while enjoying working with their parents. It is available online at www.GoodYearBooks.com.