M3N5 a - Modeling decimal numbers

M3N5. Students will understand the meaning of decimal fractions and common fractions in simple cases and apply them in problem-solving situations.
a. Understand a decimal fraction (i.e., 0.1) and a common fraction (i.e., 1/10 represent parts of a whole.

According to the GPS, decimal numbers are introduced in Grade 3. In Grade 3, though, students only consider decimal numbers only in the first decimal place (or the tenths place, if students have already learned fraction terminology). Decimal numbers to the 2nd decimal place and beyond are studied in Grade 4 and above.

I have heard many teachers who say they use money as the model for decimal numbers. However, if we are limiting decimal numbers to be discussed in Grade 3 to the first decimal place, we cannot use money as an appropriate model as money amounts are shown to the second decimal place. Thus, money as a model for decimal numbers is appropriate only starting in Grade 4, and only with decimal numbers with 2 decimal places. Some people argue, and I agree, that money is not a good model for decimal numbers.

So, why isn't money a good model for decimal numbers? First, as we saw above, it is a very limited utility as a model - only in Grade 4 (and above) and only when we are dealing with decimal numbers with 2 decimal places. Although it is true that a 0 may be annexed to a decimal number with only 1 decimal place, e.g., 0.4 = 0.40, looking at all decimal number as 2-digit decimal numbers may not be the most helpful habit to develop.

Perhaps more serious problem with money as a model is that students (and adults) don't really have to think about money amounts as decimal numbers. Rather, they are really combinations of two monetary units, dollars and cents. By using two different units, we can simply work with two whole numbers. For example, we don't consider $2.35 as two and 45 hundredths dollars. Rather, it is TWO dollars and THIRTY-FIVE cents. If we get additional $3.18, we simply add TWO and THREE dollars and THIRTY-FIVE and EIGHTEEN cents. Therefore, we are not really considering those numbers as decimal numbers - they only use notations similar to decimal numbers. Mathematically speaking, there isn't really that much difference between monetary amounts and durations expressed in hours and minutes. If you spend 2 hours and 35 minutes watching TV and 3 hours and 18 minutes playing computer games, then you wasted 2+3=5 hours and 35+18=53 minutes!

So, if money isn't a good model, what other models can we use? Base-10 blocks are always an option - we just have to designate something other than unit cubes as "1." We can also use paper strips, too, just as you might use them to model fractions. No matter what model you decide to use, an important idea we want children to develop is the unitary perspective of decimal numbers. For example, 0.4 is made up of 4 0.1-units. This is very similar to the unitary perspective of fractions I discussed in the last post. This way of looking at decimal numbers will allow students to bridge decimal numbers to whole numbers. So, it is very important for us to think about models to use, but we should also keep in mind the goal understanding we want our students to develop.