My daughter came to me for help with homework. At issue was a very odd question, it contained metric units and fractions. I discovered that she didn’t have the knowledge to convert fractions to decimals, which is the only acceptable way to do work this problem. Obviously, Common Core is attempting to address two different needs with one topic – Fractions and Metric System. This the old problem of introducing fancy, hard to pronounce names into math problems, but it doesn’t actually work. Because fractions are unacceptable with metric system or SI units.
I was able to use this as a teaching moment as was able to explain how to convert fractions to decimals and then explain why you wouldn’t want to use fractions at all.
The next day my daughter reported that her math teacher told her to use fractions, not decimals because that was activity. She also could not find any reason not to use fractions and the metric system. She even used Google and could find nothing.
The metric system uses 10 base, you can’t use 1/2 of a centimeter because it is 5 mm or .5 cm. 17/21 of a centimeter is worse. Verbally you can say “one half of a centimeter”, but this is colloquial language not a math problem.
Here is a list of websites that explain the issues:
NIST - The National Institute of Standards and Technology.
www.periodni.com, a writing style guide.
USMA – US Metric Association.
Dr. Math – an educational website.
Colorado State University, Practical Guide to SI units.
Let me sum this up, fractions are unacceptable with the metric system. It isn’t valid mathematical or in written works.