Decimals 2: Multiplication
4.5 • 1.8 = ?
Round the results to the second digit: 19.9126 becomes 19.91
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Guide for this category...

Although decimal addition is relatively simple, many are stymied when they reach multiplication, on account of the decimal point.

If you're not sure if a product looks right, consider what it would look like if the numbers after the decimal point were gone. The number before the decimal point should be similar in size to that one. If you multiply 12 * 5.1, you know that your final answer should be near 60, not 600.

The answers here should be rounded to two decimal digits, e.g. "9.12".

Rounding

You often reach a point working with decimals where the result is not practical to type out. Sometimes it's an infinite repeating number; other times it's only prohibitively long. The custom in such cases is to find a point to leave off writing. If the number after the point we've chosen is as close or closer to being 10 than 0 (5 and above), we count it as if it were a 10 and add it to our chosen cutoff point.

Now for some practical examples. Suppose we have this decimal number, 97.22222222222. If this were currency, we would like to keep track of it down to cents. (If we're accountants, we might even keep track down to mills (a tenth of a cent)). However, anything below that seems unnecessary. If we round the number to the second (decimal) digit, the number can be recorded as 97.22.

For another example, suppose we want to know how far away something is in meters. After extensive calculations, we discover the distance is 115.165747474 meters away. In this case, the thing we care about the most is before the decimal. It might be interesting to keep track of the distance down to the millimeter, but anything below that seems excessive. If we round the number off at millimeters, we get 115.166 meters. The reason why it is .166 and not .165 meters is because the number after is a 7, which is closer to 10.

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